PHYSICS. 278 



preliminary, and will have further investigation. — {Xature, xxii, 308, 

 Angust, 1880.) 



Kood has compared the color of tlio spectrum to which Xewtou gave 

 the name "indigo" with the actual color of indigo itself, and (inds it to be 

 a rei)resentative of an entirely different region of the spectrum, lie 

 shows: 1st, that the color of indigo is really a greenish blue when it is 

 used as a i)igment or in solution; 2d, that the color of the dry cake is 

 not only very black, but is variable according to the mode in which it 

 is handled. He concludes, therefore, that, taking all this into considera- 

 tion, it would appear desirable to allow the term "indigo" to fall into dis- 

 use, and to substitute for it "ultramarine," the color of the artificial variety 

 being intended, — {Am. J. ScL, III, xix, 135, February, 1880.) 



Lord Rayleigh showed to the Physical Society of London a curious 

 experiment in color combination, by mixing a blue solution of litmus 

 with a red solution of potassium dichromate, and thereby producing a 

 yellow solution. In a similar way, a colorless liquid may be obtained 

 from a green solution of cuprous chloride in water and a red solution of 

 rosaniline acetate in amyl alcohol. When both are placed in a bottle, 

 the crimson solution floats on the green solution; but on agitating them, 

 both colors disappear, the mixture becoming grayish-white, — {Xature, 

 xxii, 133, June, 1880,) 



Rood has presented to the National Academy a paper on memory for 

 color and luminosity, in which he gives the results of some interesting 

 experiments made to test the prevailing notion that we do not retain 

 for ten seconds an exact memory of a given shade or tint. The appara- 

 tus used consisted of two disks so arranged that either could be made 

 to overlap the other in any required proportion. These disks were of 

 different colors, which blended into a given tint when they were rapidly 

 revolved, a tint the percentage of whose components was known. Sup- 

 pose in one case this was 43 parts of yellow and 57 of red. After look- 

 ing at the revolving disk, an assistant disarranged the two and then 

 proceeded to reproduce the tint, Rood himself deciding when it was 

 reached. The reproduced color had 42.6 per cent, of yellow — a mean of 

 many trials, when the time of the entire experiment was not over a 

 minute. The error here was not over one-half of one per cent. When 

 an hour elapsed, the error was 2.2 per cent. Twenty-four hours after- 

 ward the error in reproducing the color was 4.5 per cent. The same 

 experiments tried by the assistant gave equally good results, as did also 

 similar experiments tried with other color mixtures. — {Xature, xxi, 144, 

 December, 1879.) 



3. Interference and Polarization. * 



Guebhard has devised a simple method of producing the phenomena. 



of Newton's rings in a permanent form. While the rings obtained are 



due to interference and give the colors of thin plates, they difl'er from 



Newton's rings proper in having the phenomena inverted, the greatest 



S. Mis. 31 18 



