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The “visibility” of x-rays
The “visibility” of x-rays John A. Reid, D.D.S.,* London, Ontario, Canada UNIVERSITY
OF WESTERN
ONTARIO
The purpose of this article is to challenge the frequently stated claim that x-rays are invisible. Whether they are, indeed, directly visible, or whether the (‘blue-gray mist? that some persons have seen is a secondary characteristic as a result of x-radiation exposure, a visual stimulation by x-radiation does occur.
I
t is generally believed and often taught that x-rays are invisible. Because this has been written so often, it has been accepted in many quarters as fact. One reason is that the wavelength of x-radiation is much below that thought necessary to stimulate the eye. Roentgen1 in his first report, ‘%oncerning a New Kind of Ray,” stated that “the retina of the eye is not sensitive to the rays,” but in 189F he reported that the rays could be seen by the unaided eye. A reference to the possibility of the eye being able to react to x-radiation when it has been dark adapted was stated in an article entitled “Vision and Oral Roentgenology” by L. R. Manson-Hing.3 Curiosity thus raised, a search of the literature mentioned in the Manson-Hing article was made and further references were turned up and examined. It is apparent that, under certain conditions, x-rays are visible. Many x-radiation workers have noticed that on occasion they have experienced a glow or sensation of light but only when the x-rays fall directly upon the retina. There is no luminosity, only the impression of “light.” Looking at a beam of x-rays but not being in that beam does not result in a sensation of light. This is probably analogous to a beam of light rays shining through a vacuum ; they are not visible until they strike some object, such as dust, that will reflect the photons from the beam of light. The concept of invisibility of x-rays probably came from the fact that one must be in the beam of radiation or it will not be seen. X-rays produce an effect on the eyes only when the latter are actually bombarded by the rays. Pirie4 notes and shows with a diagram that when an x-ray tube sends the rays onto the retina and a lead letter “T” is placed, touching the upper eyelid, in the *Associate Professor and Chairman, Division of Dental Radiology.
330
Volume 34 Number 2
“Visibility”
of x-rays
331
path of the beam, the shadow of the “T” is thrown on the retina in an upright or correct fashion; however, when one sees an object by daylight, that object is focused by the lens of the eye in an upside-down position on the retina and turned from left to right. The inverted light image on the retina is translated by the brain into its proper relationship. Objects seen with x-rays are observed in an apparent inverted position because they are not focused by the lens but, rather, are recorded on the retina in the upright position.5 This is then translated by the brain in the apparent upside-down position. I tried these experiments, using a lead strip into which a small letter “R” was formed by pin pricks. This was held touching the closed eye after 20 minutes of dark adaptation and an exposure was made. The form of the letter “R” was noted in the inverted position. Dark adaptation was found necessary in order to distinguish which parts of the retina were and were not being irradiated. However, many patients have noted when having radiographic work done that even without dark adaptation this sensation of blue-gray light was experienced. This rather interesting phenomenon was also examined by many others, including Edison6 who was investigating the possibility of using this method to enable a blind girl whose lenses were damaged but whose retina was intact to see. The stimulation of the retina by x-radiation has been used to measure the visual acuity in cataractous eyes,’ for measuring the width of eyes,8and also for locating foreign bodies (for example, glass) in the eyes.s This latter procedure was frowned upon and is no longer used. How the eye seesthe x-rays has been the subject of considerable speculation. Changes in the accommodation of the eye by atropine, diseasessuch as aphakia, absence of the lens of the eye, or cataracts did not alter the sharpness of the image pattern. Gifford and BarthlO discussed the possible physiologic action and speculated that it could be caused by fluorescence in the retina but not fluorescence in the vitreous humor, lens, or cornea by the x-rays. Himstedt and Nagel,ll and Knapp and BIchtiger,12 in the early 1930’s, showed that rays of shorter wavelength than the accepted limit of visible light of 4,000 angstrom units may produce a visual sensation when not absorbed by the tissues of the eye and that they may produce a photochemical effect on the rods. Newell and Barley’ believed that both x-ray vision and ordinary vision are similarly dependent on visual purple and that the dark adaptation curve is very similar for both light and x-rays. Rather interestingly, when repeating Pirie’s experiment and being dark adapted, I directed the beam of x-rays at a glass ash tray. I noted that it did indeed glow in the dark, but only when the beam was directed on it. A very detailed and interesting book about the history of radiation is The Trail of the Invisible Light by E. R. N. Grigg. I3 In personal correspondence with Dr. Grigg, I inquired about the title of his book. Dr. Grigg was quite aware of this phenomenon and, in his reply, said: “It was noted in 1896 that the darkadapted retina could notice that x-rays were coming through. All experimentation ceased when the damages produced by ionizing radiation upon tissue became known. It is a matter of threshold. Gamma radiation emitted by an H-bomb may carbonize organic matter. Somewhere between ‘invisible light’ and
332
Reid
Oral August,
Surg. 1972
‘carbonizing beam’ there must be a level at which the retina will respond, and when the retina responds the only thing it transmits to the cerebral cortex is a light sensation (painful if too strong, but still a light sensation). Human subjects must never be used for such experimentation, but there are experimental arrangements for measuring the electrical current produced by the excited retina, the so-called retinogram. One may use that to determine the threshold at which an x-ray beam will produce a retinogram.“14 It was found by Newell and Barley? that light is 5,000 times as efficient as x-rays for retinal stimulation, and supposedly this is the reason why there must be such long dark adaptation. Very likely the acuity for the x-ray stimulation is greatly increased after dark adaptation. Dr. Grigg states that the term &visible light originated in 1896 with the work of Morton and HammerI and of the dental radiologist, William RollinslG of Boston, who wrote the volume Notes on X-Light. It may be of particular interest that Dr. Grigg devotes a chapter in his book to the story of William James Morton, whom he describes as the “first medical electroradiologist in America” and who was also to be, above everything else, the “earliest American x-ray specialist.” His father was William Thomas Green Morton, one of the codiscoverers of anesthesia and a dentist. I7 The son, William James, received his M.D. from Harvard in 18’72. Some of his ambitions seem to have been to vindicate his father’s memory (that is, to uphold his scientific claims) and to avoid his father’s mistakes, to strive for profitable or monetary reward, and to attach chronologically indisputable claims to all his own future contributions. As an electrotherapist, Morton had devised a tube called “the Morton tube,” by which he qualified for that very select club of scientists who, however unwittingly, had produced roentgen rays long before Roentgen’s more deliberate endeavors of 1895. Before the end of 1896 Morton, together with the electrical engineer Edwin Wesley Hammer, published a paper on “The X-ray or Photography of the Invisible.“15 The experienced radiologist, H. M. Worth,l” commented as follows on the phenomenon of the visibility of x-rays: “Yes, indeed, I have often ‘seen’ x-rays. It is not often that one looks into a beam of radiation, but when one does and, even in the light, there may be an impression of some change on the retina or in the retina. As a matter of fact it was at one time tried as a method of localizing foreign bodies in the eye, the patient being instructed to indicate the site of the body as seen by looking into an x-ray beam. This was not a wise procedure. Even when having teeth radiographed, some people have said that they could ‘see’ the ray. The x-rays have to be directed into the eye, of course. Yes, it has been well known over many years. There certainly does not have to be dark adaptation. Many patients undergoing dental radiographic examination have told me that they could ‘see’ the rays.” Many of the texts on radiologic physics seem to carefully avoid listing the invisibility of x-rays as among the characteristics or properties of x-rays.‘0-21 Probably these authors are aware that x-rays can cause a visual stimulation and, as a result, they do not classify x-rays as being invisible. Interestingly, a section about “Roentgen Rays-‘Visibility,’ ” written by R. R. Newell, is included in
Volume 34 Number 2
“Visibility”
of x-rays
333
Glasser’P Medical Physics. Nevertheless, many writers of articles in journals and books have made the statement that x-rays are invisible, and it is the purpose of the present article to correct this impression among some teachers and writers. A recent interesting application of this phenomenon is the fact that the Apollo 15 astronauts conducted experiments in which they donned blindfolds, enabling them to become dark adapted, and watched for cosmic ray flashes. As reported in the newspapers, 23 these flashes and streaks of light have been seen by every Apollo crew since and including Apollo 11, and they are believed to be caused by the passage of cosmic rays through the eyeball, the retina, or the visual centers of the cerebral cortex. The three Apollo 15 crew members reported seeing these flashes with an average frequency of once every 2 minutes during an hour’s observation. The cosmic-ray particles that the NASA scientists are considering are particles with an atomic number of 6 or greater, meaning the nuclei of carbon, nitrogen, and oxygen. There is some controversy among the scientists about whether the flashes are produced by Cerenkov radiation from high-energy particles traversing the eyeball, or whether they result from collision of these particles in the retina or in the brain’s visual center. The experts are concerned that the reaction is not on the retina but, rather, with the optic nerve or the vision center of the brain because of the importance of these biologic interreactions. Calculations indicate that if the reaction occurs in the brain, it is possible that as much as 1 to 10 per cent of the cells in the brain could be damaged during a S-year mission. Therefore, the effect of the cosmic rays is extremely important for long range planning of space exploration. Space scientists report that a certain amount of adaptation to the dark may be required to see the flashes because, curiously, crews of the earlier Gemini and Mercury flights did not report seeing such flashes. Some of these astronauts have since said that they do not recall seeing the flashes; probably there were far fewer opportunities because they were too busy to notice. Their eyes were probably not dark adapted, as they were busy watching lighted instruments and, being closer in the earth’s atmosphere, the cosmic ray particles may interact only every 15 minutes or once an hour. Dr. Worth notes that he has never heard of a blind person seeing x-rays. Information from the Canadian National Institute for the Blind suggests that the major cause of blindness is macular degeneration, diabetic retinopathy, and glaucoma, and the effect of these diseases is to destroy the retina or the optic nerve. If these blind persons do not, indeed, see x-rays, then Dr. Worth suggests that anything that is seen by the astronauts due to radiation is likely to be due to effects upon the retina and not on the visual centers of the brain. The visual centers, of course, persist in blind persons; yet these persons have not, so far as is known, been aware of any influence of a visual nature while being radiographed. In addition, these persons would certainly be dark adapted and any cosmic rays would penetrate to the visual centers and should be noticeable to them. It must be emphasized that the purpose of this article is not to interest dentists in testing x-rays to produce visible stimulation and to see the “blue-
334
Reid
Oral Surg. August, 1972
gray mist” but, rather, to challenge what often is accepted as truth because it has been said and written so many times that x-rays are invisible. Whether they are directly visible, or whether the action is a secondary characteristic as a result of x-radiation exposure, nevertheless a visual stimulation by x-radiation does occur. REFERENCES 1. Roentgen,
2. 3. ;: 6. 7. 8. 9. 10. :;13: 14. 15. 16. :i* 19. 20. 21. 22. 23.
W. C.: Concerning a New Kind of Ray (Translated from German and reprinted in English)? Am. J. Roentgenol. 10: 320-320, 1923. Roentgen, W. C.: Sltzungsb. D. K. Akad. Wissensch. 1: 576, 1897. k&ransonAH~g, L. R.: Vision and Oral Roentgenology, ORAL Sua~. 16: 173-183, 1962. . . Reading With Closed Eyes, J. Can. Med. Assoc. 27: 488-490, 1932. Experiments on Vision Without Inversion of the Strat)ton; Gkb. M.: Some Preliminary Retinal Image, Psychol. Rev. 3: 611-617, 1896; 4: 341-360, 463-481, 1897. Edison, Thos. A.: Report by John Carbutt in Electrical Engineer, pp. 601-602, Dec. 9, 1896. Newell, R. R., and Borley, W. E.: Roentgen Measurements of Visual Acuity in Cataractous Eyes, Radiology, 37: 54-61, 1941. Deller, J. F. P., O’Connor, A. D., and Sorsby, A.: X-ray Measurements of the Diameters of the Living Eye, Proc. R. Sot. Lond. [Series B] 134: 456-467,1947. Pirie, A. H.: An Apparatus for Reading With Closed Eyes, Br. J. Radiol. 7: 111-116, 1934. Gifford, 8. R., and Barth, E. E.: Visual Sensation Produced by Roentgen and Radium Rays, Arch. Ophthalmol. 11: 81-91, 1934. Himstedt and Nagel: Ann. Phys. Chem. 4: 537,190l. Knapp, P., and Bachtiger, P. : Klin. Monatsbl. Augenheilkd. 89: 456,1932. Grigg, E. R. N.: The Trail of the Invisible Light, Springfield, Ill., 1965, Charles C Thomas, Publisher. Grigg, E. R. N.: Personal communication, 1971. Morton, W. J., and Hammer, E. W.: The X-ray or Photography of the Invisible and Its Value in Surgery, New York, 1896, American Technical Book Co. Rollins, Wm. : Notes on X-Light, Boston, 1904, Cambridge University Press. Raper, H. R.: Man Against Pain, New York, 1945, Prentice-Hall. Worth. II. M. : Personal communication. 1970. Johns; H. E., and Cunningham, H. R.: The Physics of Radiology, ed. 3, Springfield, Ill., 1969, Charles C Thomas, Publisher. Ter Pogossin, M.: The Physical Aspects of Diagnostic Radiology, New York, 1967, Hoeber Medical Division, Harper 6t Row. Meredith, W. J., and Massey, J. B.: Fundamental Physics of Radiology, Bristol, 1968, John Wright & Sons, Ltd. Newell. R. R.: In Glasser. 0. (editor) : Medical Phvsics, -, 1960,, Year Book MediI I Chicago. cal Publishers, Inc., pp. 661-603. ’ Spurgeon, David: Apollo XV Trio Don Blindfolds to Watch Cosmic Ray Flashes, Globe & Mail Report, July 29, 1971.
Eeprint Tepuests to : Dr. John A. Reid Division of Dental Radiology The University of Western Ontario London, Ontario, Canada
OF WESTERN
ONTARIO
The purpose of this article is to challenge the frequently stated claim that x-rays are invisible. Whether they are, indeed, directly visible, or whether the (‘blue-gray mist? that some persons have seen is a secondary characteristic as a result of x-radiation exposure, a visual stimulation by x-radiation does occur.
I
t is generally believed and often taught that x-rays are invisible. Because this has been written so often, it has been accepted in many quarters as fact. One reason is that the wavelength of x-radiation is much below that thought necessary to stimulate the eye. Roentgen1 in his first report, ‘%oncerning a New Kind of Ray,” stated that “the retina of the eye is not sensitive to the rays,” but in 189F he reported that the rays could be seen by the unaided eye. A reference to the possibility of the eye being able to react to x-radiation when it has been dark adapted was stated in an article entitled “Vision and Oral Roentgenology” by L. R. Manson-Hing.3 Curiosity thus raised, a search of the literature mentioned in the Manson-Hing article was made and further references were turned up and examined. It is apparent that, under certain conditions, x-rays are visible. Many x-radiation workers have noticed that on occasion they have experienced a glow or sensation of light but only when the x-rays fall directly upon the retina. There is no luminosity, only the impression of “light.” Looking at a beam of x-rays but not being in that beam does not result in a sensation of light. This is probably analogous to a beam of light rays shining through a vacuum ; they are not visible until they strike some object, such as dust, that will reflect the photons from the beam of light. The concept of invisibility of x-rays probably came from the fact that one must be in the beam of radiation or it will not be seen. X-rays produce an effect on the eyes only when the latter are actually bombarded by the rays. Pirie4 notes and shows with a diagram that when an x-ray tube sends the rays onto the retina and a lead letter “T” is placed, touching the upper eyelid, in the *Associate Professor and Chairman, Division of Dental Radiology.
330
Volume 34 Number 2
“Visibility”
of x-rays
331
path of the beam, the shadow of the “T” is thrown on the retina in an upright or correct fashion; however, when one sees an object by daylight, that object is focused by the lens of the eye in an upside-down position on the retina and turned from left to right. The inverted light image on the retina is translated by the brain into its proper relationship. Objects seen with x-rays are observed in an apparent inverted position because they are not focused by the lens but, rather, are recorded on the retina in the upright position.5 This is then translated by the brain in the apparent upside-down position. I tried these experiments, using a lead strip into which a small letter “R” was formed by pin pricks. This was held touching the closed eye after 20 minutes of dark adaptation and an exposure was made. The form of the letter “R” was noted in the inverted position. Dark adaptation was found necessary in order to distinguish which parts of the retina were and were not being irradiated. However, many patients have noted when having radiographic work done that even without dark adaptation this sensation of blue-gray light was experienced. This rather interesting phenomenon was also examined by many others, including Edison6 who was investigating the possibility of using this method to enable a blind girl whose lenses were damaged but whose retina was intact to see. The stimulation of the retina by x-radiation has been used to measure the visual acuity in cataractous eyes,’ for measuring the width of eyes,8and also for locating foreign bodies (for example, glass) in the eyes.s This latter procedure was frowned upon and is no longer used. How the eye seesthe x-rays has been the subject of considerable speculation. Changes in the accommodation of the eye by atropine, diseasessuch as aphakia, absence of the lens of the eye, or cataracts did not alter the sharpness of the image pattern. Gifford and BarthlO discussed the possible physiologic action and speculated that it could be caused by fluorescence in the retina but not fluorescence in the vitreous humor, lens, or cornea by the x-rays. Himstedt and Nagel,ll and Knapp and BIchtiger,12 in the early 1930’s, showed that rays of shorter wavelength than the accepted limit of visible light of 4,000 angstrom units may produce a visual sensation when not absorbed by the tissues of the eye and that they may produce a photochemical effect on the rods. Newell and Barley’ believed that both x-ray vision and ordinary vision are similarly dependent on visual purple and that the dark adaptation curve is very similar for both light and x-rays. Rather interestingly, when repeating Pirie’s experiment and being dark adapted, I directed the beam of x-rays at a glass ash tray. I noted that it did indeed glow in the dark, but only when the beam was directed on it. A very detailed and interesting book about the history of radiation is The Trail of the Invisible Light by E. R. N. Grigg. I3 In personal correspondence with Dr. Grigg, I inquired about the title of his book. Dr. Grigg was quite aware of this phenomenon and, in his reply, said: “It was noted in 1896 that the darkadapted retina could notice that x-rays were coming through. All experimentation ceased when the damages produced by ionizing radiation upon tissue became known. It is a matter of threshold. Gamma radiation emitted by an H-bomb may carbonize organic matter. Somewhere between ‘invisible light’ and
332
Reid
Oral August,
Surg. 1972
‘carbonizing beam’ there must be a level at which the retina will respond, and when the retina responds the only thing it transmits to the cerebral cortex is a light sensation (painful if too strong, but still a light sensation). Human subjects must never be used for such experimentation, but there are experimental arrangements for measuring the electrical current produced by the excited retina, the so-called retinogram. One may use that to determine the threshold at which an x-ray beam will produce a retinogram.“14 It was found by Newell and Barley? that light is 5,000 times as efficient as x-rays for retinal stimulation, and supposedly this is the reason why there must be such long dark adaptation. Very likely the acuity for the x-ray stimulation is greatly increased after dark adaptation. Dr. Grigg states that the term &visible light originated in 1896 with the work of Morton and HammerI and of the dental radiologist, William RollinslG of Boston, who wrote the volume Notes on X-Light. It may be of particular interest that Dr. Grigg devotes a chapter in his book to the story of William James Morton, whom he describes as the “first medical electroradiologist in America” and who was also to be, above everything else, the “earliest American x-ray specialist.” His father was William Thomas Green Morton, one of the codiscoverers of anesthesia and a dentist. I7 The son, William James, received his M.D. from Harvard in 18’72. Some of his ambitions seem to have been to vindicate his father’s memory (that is, to uphold his scientific claims) and to avoid his father’s mistakes, to strive for profitable or monetary reward, and to attach chronologically indisputable claims to all his own future contributions. As an electrotherapist, Morton had devised a tube called “the Morton tube,” by which he qualified for that very select club of scientists who, however unwittingly, had produced roentgen rays long before Roentgen’s more deliberate endeavors of 1895. Before the end of 1896 Morton, together with the electrical engineer Edwin Wesley Hammer, published a paper on “The X-ray or Photography of the Invisible.“15 The experienced radiologist, H. M. Worth,l” commented as follows on the phenomenon of the visibility of x-rays: “Yes, indeed, I have often ‘seen’ x-rays. It is not often that one looks into a beam of radiation, but when one does and, even in the light, there may be an impression of some change on the retina or in the retina. As a matter of fact it was at one time tried as a method of localizing foreign bodies in the eye, the patient being instructed to indicate the site of the body as seen by looking into an x-ray beam. This was not a wise procedure. Even when having teeth radiographed, some people have said that they could ‘see’ the ray. The x-rays have to be directed into the eye, of course. Yes, it has been well known over many years. There certainly does not have to be dark adaptation. Many patients undergoing dental radiographic examination have told me that they could ‘see’ the rays.” Many of the texts on radiologic physics seem to carefully avoid listing the invisibility of x-rays as among the characteristics or properties of x-rays.‘0-21 Probably these authors are aware that x-rays can cause a visual stimulation and, as a result, they do not classify x-rays as being invisible. Interestingly, a section about “Roentgen Rays-‘Visibility,’ ” written by R. R. Newell, is included in
Volume 34 Number 2
“Visibility”
of x-rays
333
Glasser’P Medical Physics. Nevertheless, many writers of articles in journals and books have made the statement that x-rays are invisible, and it is the purpose of the present article to correct this impression among some teachers and writers. A recent interesting application of this phenomenon is the fact that the Apollo 15 astronauts conducted experiments in which they donned blindfolds, enabling them to become dark adapted, and watched for cosmic ray flashes. As reported in the newspapers, 23 these flashes and streaks of light have been seen by every Apollo crew since and including Apollo 11, and they are believed to be caused by the passage of cosmic rays through the eyeball, the retina, or the visual centers of the cerebral cortex. The three Apollo 15 crew members reported seeing these flashes with an average frequency of once every 2 minutes during an hour’s observation. The cosmic-ray particles that the NASA scientists are considering are particles with an atomic number of 6 or greater, meaning the nuclei of carbon, nitrogen, and oxygen. There is some controversy among the scientists about whether the flashes are produced by Cerenkov radiation from high-energy particles traversing the eyeball, or whether they result from collision of these particles in the retina or in the brain’s visual center. The experts are concerned that the reaction is not on the retina but, rather, with the optic nerve or the vision center of the brain because of the importance of these biologic interreactions. Calculations indicate that if the reaction occurs in the brain, it is possible that as much as 1 to 10 per cent of the cells in the brain could be damaged during a S-year mission. Therefore, the effect of the cosmic rays is extremely important for long range planning of space exploration. Space scientists report that a certain amount of adaptation to the dark may be required to see the flashes because, curiously, crews of the earlier Gemini and Mercury flights did not report seeing such flashes. Some of these astronauts have since said that they do not recall seeing the flashes; probably there were far fewer opportunities because they were too busy to notice. Their eyes were probably not dark adapted, as they were busy watching lighted instruments and, being closer in the earth’s atmosphere, the cosmic ray particles may interact only every 15 minutes or once an hour. Dr. Worth notes that he has never heard of a blind person seeing x-rays. Information from the Canadian National Institute for the Blind suggests that the major cause of blindness is macular degeneration, diabetic retinopathy, and glaucoma, and the effect of these diseases is to destroy the retina or the optic nerve. If these blind persons do not, indeed, see x-rays, then Dr. Worth suggests that anything that is seen by the astronauts due to radiation is likely to be due to effects upon the retina and not on the visual centers of the brain. The visual centers, of course, persist in blind persons; yet these persons have not, so far as is known, been aware of any influence of a visual nature while being radiographed. In addition, these persons would certainly be dark adapted and any cosmic rays would penetrate to the visual centers and should be noticeable to them. It must be emphasized that the purpose of this article is not to interest dentists in testing x-rays to produce visible stimulation and to see the “blue-
334
Reid
Oral Surg. August, 1972
gray mist” but, rather, to challenge what often is accepted as truth because it has been said and written so many times that x-rays are invisible. Whether they are directly visible, or whether the action is a secondary characteristic as a result of x-radiation exposure, nevertheless a visual stimulation by x-radiation does occur. REFERENCES 1. Roentgen,
2. 3. ;: 6. 7. 8. 9. 10. :;13: 14. 15. 16. :i* 19. 20. 21. 22. 23.
W. C.: Concerning a New Kind of Ray (Translated from German and reprinted in English)? Am. J. Roentgenol. 10: 320-320, 1923. Roentgen, W. C.: Sltzungsb. D. K. Akad. Wissensch. 1: 576, 1897. k&ransonAH~g, L. R.: Vision and Oral Roentgenology, ORAL Sua~. 16: 173-183, 1962. . . Reading With Closed Eyes, J. Can. Med. Assoc. 27: 488-490, 1932. Experiments on Vision Without Inversion of the Strat)ton; Gkb. M.: Some Preliminary Retinal Image, Psychol. Rev. 3: 611-617, 1896; 4: 341-360, 463-481, 1897. Edison, Thos. A.: Report by John Carbutt in Electrical Engineer, pp. 601-602, Dec. 9, 1896. Newell, R. R., and Borley, W. E.: Roentgen Measurements of Visual Acuity in Cataractous Eyes, Radiology, 37: 54-61, 1941. Deller, J. F. P., O’Connor, A. D., and Sorsby, A.: X-ray Measurements of the Diameters of the Living Eye, Proc. R. Sot. Lond. [Series B] 134: 456-467,1947. Pirie, A. H.: An Apparatus for Reading With Closed Eyes, Br. J. Radiol. 7: 111-116, 1934. Gifford, 8. R., and Barth, E. E.: Visual Sensation Produced by Roentgen and Radium Rays, Arch. Ophthalmol. 11: 81-91, 1934. Himstedt and Nagel: Ann. Phys. Chem. 4: 537,190l. Knapp, P., and Bachtiger, P. : Klin. Monatsbl. Augenheilkd. 89: 456,1932. Grigg, E. R. N.: The Trail of the Invisible Light, Springfield, Ill., 1965, Charles C Thomas, Publisher. Grigg, E. R. N.: Personal communication, 1971. Morton, W. J., and Hammer, E. W.: The X-ray or Photography of the Invisible and Its Value in Surgery, New York, 1896, American Technical Book Co. Rollins, Wm. : Notes on X-Light, Boston, 1904, Cambridge University Press. Raper, H. R.: Man Against Pain, New York, 1945, Prentice-Hall. Worth. II. M. : Personal communication. 1970. Johns; H. E., and Cunningham, H. R.: The Physics of Radiology, ed. 3, Springfield, Ill., 1969, Charles C Thomas, Publisher. Ter Pogossin, M.: The Physical Aspects of Diagnostic Radiology, New York, 1967, Hoeber Medical Division, Harper 6t Row. Meredith, W. J., and Massey, J. B.: Fundamental Physics of Radiology, Bristol, 1968, John Wright & Sons, Ltd. Newell. R. R.: In Glasser. 0. (editor) : Medical Phvsics, -, 1960,, Year Book MediI I Chicago. cal Publishers, Inc., pp. 661-603. ’ Spurgeon, David: Apollo XV Trio Don Blindfolds to Watch Cosmic Ray Flashes, Globe & Mail Report, July 29, 1971.
Eeprint Tepuests to : Dr. John A. Reid Division of Dental Radiology The University of Western Ontario London, Ontario, Canada