- Email: [email protected]
Description of a new universal photometer
lab
P@sical Science.
collecting our thoughts, of fixing with precision all the facts, of bringing together and summing up the opinions which explain and deveiope these great principles; lastly, that it was useful, as regarded your future studies, to give y o u in writing, and in a dearer form, the expression of these views, which were partly brought into existence under the stimulus of y o u r presence, and consequently reduced into tbrm with the hesitation which so often accompany the first enunciation of our thoughts. [TO JJ~: cox'rL~ bq~Ja.]
Description o f a )~cw [5ziversal Photometer. B j Dr,. CmXRL;SS Scm,~rnAm;'rl., ¢f 31ttnich, dlssoe. Insl. C. E. The inadequacy of' the photonxctric instalments invented b y Pietet, Rumford, and others, is universally acknowledged. The bromide o f silver, its used by Sir John. 1 [ersehell, although extremely sensitive, is only slighlly afl'eetcd by artificial light. These ciremnstanecs induced tim author to complete the present instrument, which hc contenli)/ated about twelve years sit~ce. The intensity oi'tlm tmdulations el'gaseous fluid.% as well as that of the air, is proportio~ml to tim amplitude of tim oscillations, or, more properly, to ~he square of the ~mptitude. A w a v e of ~.zhl striking the 1'/2[[111t.tlll]St create a similar vibratory motion in lho ncrrcs of the retina, be.cause lhe velocity of tile molecular movement of the nerves depends upoll the force with which they have been struck by the original w a v e , and if this velocity could be measured, it would show at the same time the intensity o,~"light. It is scarcely possible to obtain a direct accurate measurement o f this velocity, but if the time during which the vibratory motion of the nerves ceases be ascertained, the velocity of the vibrating molecules, and, therefor% the intensity of light, m a y be determined, because tile duration of an impression on the retina is dependent on the resistance which the molecules of tl m nerves oppose to every three striking them; but as this resistance of tile nerves increases as the square of the velocity, tbur times the n~omcntum, or i~tensfty, is necessary to double the time of dm'ation--or, in other words, tile intensity of the pencil of rays is as the s~luare of the time of the duration of that impression m a d e on tile nerves of the reliua. The n e w photometer consists of a brass bar fixed vertically in a stand, carrying at ils upper end a small tabe in two parts, which m a y be lengthened from five to ten inches, if requisite. This eye tube has at each end a sliding plate pierced with holes of corresponding diameters. F r o m the bottom of the bar a projecting arm sustains tile lower end of a strip of rolled steel, eighteen inches lena, -rsvths inch broad, and @~nd inch thick; this has at the upper end a t h m plate, pierced with a small hole, corresponding with the. holes in the sliders, and standing one-eighth of an inch fl'om one of them: upon tile main bar is a prism with a slit in it, through which the strip of steel passes; this prism can be m o v e d up or down, b y a rack and pinion~ so as to lengthen or shorten the vibrations of the strip.
PropagaHon of Sound.
~9
The method of using the instrument is to adjust the two holes a~ tim opposite ends of the horizontal eye tube, so that they perfectl)" correspond, and do not permit any ray~ of light to enter, unless tlm plate at the extremity of the spring, be pushed aside. The light to be compared is then placed at a certain glver~ distance behind the plate~ so that by bringing the axis of the hole which is pierced in it into 1he axis of the tube, a small pencil of light may enter the pupil of the eye. The prism is then placed at 100 of the scale • on the side of the brass bar, and the steel strip caused to vibrate gently. A luminous disk immediately appears, accompanied by seintillatfons, which are caused by the impressions on the retina being interrupted by dark intervals: the prism is then gradually raised, until the length of the vibrations of the strip being diminished, and the velocity increased', the luminous disk a ~pea,s peileetly steady and clear the lenath of the vibraling portion of the strip is then react off by the verniers marked on the brass rod, and compared with the whole length of the spring, measured /i'om 100, w-hich is considered as unity. The number of the vibrations to be computed f?om the tbund length of the spring, are inversely to the numbers of vibrations of the whole lengeh, as ~he. squares of their relative lengths. Hence are constructed the formulm. tbr calculation, which are given at length in the communication. A fresh luminous impression is made on the retina as often as the circular aperture ia the screen, on the top of the spring, cuts the axis of the tube. If" the duration of the small vibration of the nerves of the retina is shorter than *lle time of a vibration of the spring, a dark interval appears between the two luminoasimpressions. In this ease the vibration of the spring is shortened until the next impressiml retuttis just as the first ceases, and therefore the dark interval disappears ; then by measuring the length of the shortened spring, the num,ber of vibrations can be computed, and fl'om them the intensity of the -
~
,"
light.=--Tra~zs, of the Inst. of Civ. Eng..
r
aour. of A r t s & Set. Nov.
Propagatio~ of 8ound... M. Collation has fbrwarded to the French Academ.y o+fSciences all, acco.unt of some experiments he had been making on the Lake of Geneva, relative to the propagation of sound; he had tried ov.er agail~ an experiment made by N?r. Bonnyeastle, oa the coast of the United States, in lSaS, and recornmendod by M. Arago, ia~ order ta find whether sotmd could be reflected back fl'om the bottom, of a lake, or piece of water, and by measuring the tiine of its passage, to calculate the depth of the water. Mr. :Bonnycastle had not been. able to.hear the smmd of a bell under water at a greater distance than 8000 or 10~000; • feet, but M. Colladon had succeeded i.n propagating a sound of this kind to the distance of 13.,500 metres, or 42,640 feet; he. had also, fomad that when a blow was struck on a bell partly out of water and partly under water, two sout~s were heard, one coming by the air, and the other •by the water. At a little, distance the latter was the less stroog of the tree, but at a great distance the contrary was the 3.
P@sical Science.
collecting our thoughts, of fixing with precision all the facts, of bringing together and summing up the opinions which explain and deveiope these great principles; lastly, that it was useful, as regarded your future studies, to give y o u in writing, and in a dearer form, the expression of these views, which were partly brought into existence under the stimulus of y o u r presence, and consequently reduced into tbrm with the hesitation which so often accompany the first enunciation of our thoughts. [TO JJ~: cox'rL~ bq~Ja.]
Description o f a )~cw [5ziversal Photometer. B j Dr,. CmXRL;SS Scm,~rnAm;'rl., ¢f 31ttnich, dlssoe. Insl. C. E. The inadequacy of' the photonxctric instalments invented b y Pietet, Rumford, and others, is universally acknowledged. The bromide o f silver, its used by Sir John. 1 [ersehell, although extremely sensitive, is only slighlly afl'eetcd by artificial light. These ciremnstanecs induced tim author to complete the present instrument, which hc contenli)/ated about twelve years sit~ce. The intensity oi'tlm tmdulations el'gaseous fluid.% as well as that of the air, is proportio~ml to tim amplitude of tim oscillations, or, more properly, to ~he square of the ~mptitude. A w a v e of ~.zhl striking the 1'/2[[111t.tlll]St create a similar vibratory motion in lho ncrrcs of the retina, be.cause lhe velocity of tile molecular movement of the nerves depends upoll the force with which they have been struck by the original w a v e , and if this velocity could be measured, it would show at the same time the intensity o,~"light. It is scarcely possible to obtain a direct accurate measurement o f this velocity, but if the time during which the vibratory motion of the nerves ceases be ascertained, the velocity of the vibrating molecules, and, therefor% the intensity of light, m a y be determined, because tile duration of an impression on the retina is dependent on the resistance which the molecules of tl m nerves oppose to every three striking them; but as this resistance of tile nerves increases as the square of the velocity, tbur times the n~omcntum, or i~tensfty, is necessary to double the time of dm'ation--or, in other words, tile intensity of the pencil of rays is as the s~luare of the time of the duration of that impression m a d e on tile nerves of the reliua. The n e w photometer consists of a brass bar fixed vertically in a stand, carrying at ils upper end a small tabe in two parts, which m a y be lengthened from five to ten inches, if requisite. This eye tube has at each end a sliding plate pierced with holes of corresponding diameters. F r o m the bottom of the bar a projecting arm sustains tile lower end of a strip of rolled steel, eighteen inches lena, -rsvths inch broad, and @~nd inch thick; this has at the upper end a t h m plate, pierced with a small hole, corresponding with the. holes in the sliders, and standing one-eighth of an inch fl'om one of them: upon tile main bar is a prism with a slit in it, through which the strip of steel passes; this prism can be m o v e d up or down, b y a rack and pinion~ so as to lengthen or shorten the vibrations of the strip.
PropagaHon of Sound.
~9
The method of using the instrument is to adjust the two holes a~ tim opposite ends of the horizontal eye tube, so that they perfectl)" correspond, and do not permit any ray~ of light to enter, unless tlm plate at the extremity of the spring, be pushed aside. The light to be compared is then placed at a certain glver~ distance behind the plate~ so that by bringing the axis of the hole which is pierced in it into 1he axis of the tube, a small pencil of light may enter the pupil of the eye. The prism is then placed at 100 of the scale • on the side of the brass bar, and the steel strip caused to vibrate gently. A luminous disk immediately appears, accompanied by seintillatfons, which are caused by the impressions on the retina being interrupted by dark intervals: the prism is then gradually raised, until the length of the vibrations of the strip being diminished, and the velocity increased', the luminous disk a ~pea,s peileetly steady and clear the lenath of the vibraling portion of the strip is then react off by the verniers marked on the brass rod, and compared with the whole length of the spring, measured /i'om 100, w-hich is considered as unity. The number of the vibrations to be computed f?om the tbund length of the spring, are inversely to the numbers of vibrations of the whole lengeh, as ~he. squares of their relative lengths. Hence are constructed the formulm. tbr calculation, which are given at length in the communication. A fresh luminous impression is made on the retina as often as the circular aperture ia the screen, on the top of the spring, cuts the axis of the tube. If" the duration of the small vibration of the nerves of the retina is shorter than *lle time of a vibration of the spring, a dark interval appears between the two luminoasimpressions. In this ease the vibration of the spring is shortened until the next impressiml retuttis just as the first ceases, and therefore the dark interval disappears ; then by measuring the length of the shortened spring, the num,ber of vibrations can be computed, and fl'om them the intensity of the -
~
,"
light.=--Tra~zs, of the Inst. of Civ. Eng..
r
aour. of A r t s & Set. Nov.
Propagatio~ of 8ound... M. Collation has fbrwarded to the French Academ.y o+fSciences all, acco.unt of some experiments he had been making on the Lake of Geneva, relative to the propagation of sound; he had tried ov.er agail~ an experiment made by N?r. Bonnyeastle, oa the coast of the United States, in lSaS, and recornmendod by M. Arago, ia~ order ta find whether sotmd could be reflected back fl'om the bottom, of a lake, or piece of water, and by measuring the tiine of its passage, to calculate the depth of the water. Mr. :Bonnycastle had not been. able to.hear the smmd of a bell under water at a greater distance than 8000 or 10~000; • feet, but M. Colladon had succeeded i.n propagating a sound of this kind to the distance of 13.,500 metres, or 42,640 feet; he. had also, fomad that when a blow was struck on a bell partly out of water and partly under water, two sout~s were heard, one coming by the air, and the other •by the water. At a little, distance the latter was the less stroog of the tree, but at a great distance the contrary was the 3.