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The application of optical methods to the examination of works of art
C U R R E N T TOPICS. T h e Photoluminescence of Flames. E . L . NICHOLS and H. L. HOWES. (Phys. Rev., Nov., 1923.)--" The brightness of the flame plus the light transmitted by the flame from a nitrogen-filled tungsten lamp placed behind it, should normally be slightly less than the sum of the two intensities taken separately, whereas instead of showing a loss by absorption the combined brightness was persistently greater." P~fter failure of other explanations that suggested themselves the authors were driven to conclude " t h a t the flame was actually somewhat brighter when exposed to the light of the neighboring source." They proceeded to test this hypothesis and found that the facts agreed with it. An air-hydrogen blast lamp was used with Li, Na, Sr and Ca salts. Light from a tungsten lamp, a carbon arc, a mercury arc and an iron spark fell in turn upon the flames. " T h e iron spark was most effective, the increases being over IO per cent. for Ca and St. Not only the ultra-violet beyond .3/~ is effective but also light above •43~." On the other hand, red or yellow light of such wave-length that it was absorbed by the flame reduced the emission of the flame by a few per cent. No matter whether the effect of the incident light was to enhance or to reduce the emission, it seemed to produce the same effect upon all the lines derived from the flame. When a calcium band was observed while its parent flame was illuminated both by light through ruby glass and by light from an iron spark its brightness was greater than that of the flame alone but less than that of the same flame exposed to the spark light alone. The flame of the Bunsen burner showed scarcely any change due to illumination, and the different parts of the hydrogen flame were sensitive in different degrees. G.F.S. Q
T h e Application of Optical Methods to the E x a m i n a t i o n of W o r k s of Art. E. BAYLE and HENRI GEORGE. (Comptes Nendus, Jan. 2, I 9 2 4 . ) - - F o r the production of light of a desired composition a projector of four beams is used. Three of these have respectively red, blue and green screens. The fourth has white light. By a combination of these beams a great number of shades of colored light may be had. By the use of the proper shade peculiarities of the painting may be photographed. " It is thus that we have been able to get very clear evidence in the case of an Impressionist master of the form of the strokes imposed upon him by a physical disability." By the employment of an ultra-violet light, which makes varnish fluoresce, the quantity of this material, as well as particular treatment, is revealed. By this means a picture signed " Goya a. 1799 " was discovered to have another inscription to the effect that it was a copy. Spectroscopic methods are used in the study of the colors on the painting. Mercury is thus determined and one-fourth mg. of paint suffices 436
March, 1924.]
CURRENT TOpLCS.
437
to bring out lead lines in the spark. The microscope is valuable in the examination of mixed pigments. The spectrophotometric composition of the light diffused from Prussian blue, cobalt blue, madder, etc., is known. These methods are supplemented by the information furnished by X-rays. G.F.S. AT THE meeting of the British Association for the Advancement of Science in 1923 Sections A, B, and G united in a joint discussion of cohesion and molecular forces. Very appropriately Sir William Bragg took up the matter of crystal structure. The old view that atoms and molecules exert in all directions forces that vary with some power of the distance has had success in explaining certain phenomena but has largely failed with solids. " A crystal conforms so exactly to rules respecting its angular dimensions that it seems impossible to imagine its form to be merely the result of an average of tendencies. The forces of adjustment cannot, therefore, be thought of as a force between two points each representing one of the molecules. On the contrary, it is nearer the truth to think that the adjustment is made so as to bring together certain points on one molecule and certain points on the other. In considering, therefore, the binding of the individual molecules of a solid, the analogy of the attraction of two charged spheres is imperfect, and should be replaced by that of two members of a girder structure adjusted until the rivets can be dropped into the holes brought into true alignment . . . . One well-known fact in crystal growth is that the faces have different rates of growth, indicating that there may be great differences in the ease with which molecules slip into their places." After the subject of cohesion had been discussed by several speakers it was clear that no more than a beginning has been made. " There was point in the somewhat facetious remark of Sir Oliver Lodge that it was an extraordinary fact that, after all these years, three important sections of the British Association should be gathered together to discuss why, when one end of a stick is raised from the table, the rest of it also comes up." (Nature, Nov. 24, 1923. ) G. F. S. T h e P r o d u c t i o n of an E l e c t r o m o t i v e F o r c e b y Illumination. P. H. GEIGER. (Phys. Rev., Nov., I 9 2 3 . ) - - W h e n a piece of argenrite, Ag2S, is put in circuit with a galvanometer and one contact surface of the mineral is illuminated, an electromotive force is produced. Illumination of the other face reverses the direction of the current. Specimens from different localities all showed this property. The electromotive force increases with the intensity of illumination until a maximum of .oI 3 volt is reached. The effect depends on the wavelength of the incident light, the maximum result being attained for a wave-length of ltd. Six other minerals gave the effect to some extent, while others, in which illumination produces a change of electrical resistance, failed to show any electromotive force. G. F. S.
T h e Application of Optical Methods to the E x a m i n a t i o n of W o r k s of Art. E. BAYLE and HENRI GEORGE. (Comptes Nendus, Jan. 2, I 9 2 4 . ) - - F o r the production of light of a desired composition a projector of four beams is used. Three of these have respectively red, blue and green screens. The fourth has white light. By a combination of these beams a great number of shades of colored light may be had. By the use of the proper shade peculiarities of the painting may be photographed. " It is thus that we have been able to get very clear evidence in the case of an Impressionist master of the form of the strokes imposed upon him by a physical disability." By the employment of an ultra-violet light, which makes varnish fluoresce, the quantity of this material, as well as particular treatment, is revealed. By this means a picture signed " Goya a. 1799 " was discovered to have another inscription to the effect that it was a copy. Spectroscopic methods are used in the study of the colors on the painting. Mercury is thus determined and one-fourth mg. of paint suffices 436
March, 1924.]
CURRENT TOpLCS.
437
to bring out lead lines in the spark. The microscope is valuable in the examination of mixed pigments. The spectrophotometric composition of the light diffused from Prussian blue, cobalt blue, madder, etc., is known. These methods are supplemented by the information furnished by X-rays. G.F.S. AT THE meeting of the British Association for the Advancement of Science in 1923 Sections A, B, and G united in a joint discussion of cohesion and molecular forces. Very appropriately Sir William Bragg took up the matter of crystal structure. The old view that atoms and molecules exert in all directions forces that vary with some power of the distance has had success in explaining certain phenomena but has largely failed with solids. " A crystal conforms so exactly to rules respecting its angular dimensions that it seems impossible to imagine its form to be merely the result of an average of tendencies. The forces of adjustment cannot, therefore, be thought of as a force between two points each representing one of the molecules. On the contrary, it is nearer the truth to think that the adjustment is made so as to bring together certain points on one molecule and certain points on the other. In considering, therefore, the binding of the individual molecules of a solid, the analogy of the attraction of two charged spheres is imperfect, and should be replaced by that of two members of a girder structure adjusted until the rivets can be dropped into the holes brought into true alignment . . . . One well-known fact in crystal growth is that the faces have different rates of growth, indicating that there may be great differences in the ease with which molecules slip into their places." After the subject of cohesion had been discussed by several speakers it was clear that no more than a beginning has been made. " There was point in the somewhat facetious remark of Sir Oliver Lodge that it was an extraordinary fact that, after all these years, three important sections of the British Association should be gathered together to discuss why, when one end of a stick is raised from the table, the rest of it also comes up." (Nature, Nov. 24, 1923. ) G. F. S. T h e P r o d u c t i o n of an E l e c t r o m o t i v e F o r c e b y Illumination. P. H. GEIGER. (Phys. Rev., Nov., I 9 2 3 . ) - - W h e n a piece of argenrite, Ag2S, is put in circuit with a galvanometer and one contact surface of the mineral is illuminated, an electromotive force is produced. Illumination of the other face reverses the direction of the current. Specimens from different localities all showed this property. The electromotive force increases with the intensity of illumination until a maximum of .oI 3 volt is reached. The effect depends on the wavelength of the incident light, the maximum result being attained for a wave-length of ltd. Six other minerals gave the effect to some extent, while others, in which illumination produces a change of electrical resistance, failed to show any electromotive force. G. F. S.