482 SCIENTIFIC RECORD FOR 1882. 



2. Reflection and Refraction. 



Chardonnet has made a series of experiments on the actinic trans- 

 parency of certain media, especially of the mirrors of Foucault. Two 

 methods were used : the first consisted in exposing sensitive paper to 

 the solar rays after traversing the medium ; the second required a lens 

 of quartz and Iceland spar and a prism of spar, the spectrum being re- 

 ceived on a photographic plate or a fluorescent screen. In this way the 

 various kinds of optical glass were studied, the results of which are 

 given. Silv^ered-glass mirrors transmit the more refrangible rays, giving 

 on the photographic plate a band from O to T ; thus acting as a filter, 

 allowing obscure rays only to go through. In this way the author 

 obtained photographs in absolute darkness. {J. Phys.. July, II, i, p. 305.) 



Subsequently Chardonnet has examined the spectrum of light re- 

 flected from various bodies, and comes to the conclusion that absolute 

 elective absorption does not exist; lampblack itself, deposited as an 

 opaque layer on a plate of enamel, giving a complete spectrum. The 

 substances tried were: white and black enamel, uranium glass, rough 

 hematite, polished hematite, diamond, compressed carbon, rough and pol- 

 ished, vermilion, gold, lead, nickel, Arcet's alloy, copper, rough and pol- 

 ished steel, Prussian blue, green leaves, and mercury. Among liquids 

 distilled water, fuchsine, acetosulphate of quinine, ammonia, copper sul- 

 phate, potassium dichromate, milk, ink, alcohol, ether, benzene, olive 

 oil; all of which gave a complete spectrum. The author concludes (1) 

 every surface reflects in variable proportions all the colors of the spec- 

 trum ; (2) the reflecting power of a liquid is independent of the sub- 

 stances which it holds in solution or in suspension; and (3) specular 

 polish increases only the total quantity of reflected radiations, the rela- 

 tive intensity of the different regions of the spectrum depending upon 

 the material employed. [J. Phys., December, II, i, p. 549.) 



Piltschikoff has suggested the use of a hollow lens for measuring the 

 refractive index of small quantities of liquid. After filling the lens with 

 the liquid to be examined, the focal distance of the image of a monochro- 

 matic flame, placed at a given distance from the lens, is exactly meas- 

 ured by means of a graduated rule. The constants of the apparatus 

 are determined once for all, and then, by means of a simple formula, the 

 index of the liquid is easily calculated. The results are exact, the index 

 of glycerin thus determined being 1.47298 dL .00001. [J. Phys., Decem- 

 ber, II, I, p. 578.) 



Boys has suggested a method of measuring the curvature of lenses 

 with a view to determine their refractive index. The principle of the 

 method is illustrated as follows: Fix an ordinary spectacle lens in a 

 clip with its principal plane vertical; in front of it place a card with a 

 small hole in it, and illuminate the hole with a candle flame. It will be 

 found that, when the lens is at a certain distance from the card, there is 

 an inverted image of the hole formed on the card. When this is the 



