482 SCIENTIFIC RECORD FOR 1882. 



2. Beflection and Refraction. 



Cbardonnet lias made a series of experiments on the actinic trans- 

 parency of certain media, especially of the mirrors of Foucaiilt. 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 photogra]>hic plate or a fluorescent screen. In this way the 

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

 given. Silvered-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 comjdete 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, acetosuli)hate of quinine, ammonia, coi)per 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 

 lK)lish 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.) 



Piltschikofi" 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 giveu 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 i .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 i)rincii)le 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 



