PHYSICS. 487 



combination is four-sided, so that it not only rectifies the ray which has 

 been deflected by the first prism but also increases by a nearly equal 

 amount the dispersion of the first prism. {Nature, December, xxvii, 

 p. 182.) 



Wead has proposed a method of combining color disks by the rota- 

 tion of a plane mirror slightly inclined to the axis around which it ro- 

 tates. If the angle of inclination, the distances of the eye and the 

 disks from the mirror, and the sizes of the mirror and the disk are 

 properly proportioned, a good combination of the colors may be effected 

 and the necessary adjustment of colors may be made without stopping 

 the rotation. {Nature, January, xxv, p. 2CG.) 



For the mixture of spectrum colors, von Frey and von Kries illumi- 

 nate two neighboring surfaces, one with a mixture of two spectrum 

 colors, the other either by a mixture of white light and a given spec- 

 trum color, or by white light alone. Their results for complementary 

 colors confirm those of Helmholtz. If a curve be constructed with the 

 wave-lengths of certain colors as abscissas and wave-lengths of their 

 complemehtaries as ordinates, the two branches of an equilateral hy- 

 perbola are obtained. But the curves obtained by the two observers 

 under the same conditions are not identical, but intersect in the yellow- 

 ish -blue. Analogous differences ajipear in the experiments made to 

 reproduce the different tints of the spectrum by the mixture of two 

 spectrum colors. Thus, when the red of C and the green of b are used 

 to reproduce intermediate tints, von Kries must employ more green 

 than von Frey, and less when the green of b is mixed with the violet of 

 G. These results show the eye of one of these authors to be more 

 sensitive to yellow than the other. They attribute it to unequal ab- 

 sorption by the pigment of the yeUow sjiot. {J. Phys., November, II, 

 I, p. 513.) 



Hastings has published a paper on the color correction of double ob- 

 jectives, in which, after a review of the methods heretofore employed for 

 attaining this end, he gives a theoretical solution of the problem, shows 

 how the results may be applied to the practical construction of an ob- 

 jective, and, finally, gives the resultsof some practical tests of the theory. 

 Two forms of test were applied. In the one three objectives were con- 

 structed from the theory and their performance studied. In the other 

 objectives of the most approved makers were examined and their prac- 

 tice compared with theory. The results in both cases were regarded 

 as a decisive proof of the correctness of the theory. {Am. J. Sci., 

 March, III, xxiii, p. 167.) 



4. Interference and Polarization. 



Michelson has investigated the interference phenomena observed in 

 the form of refractometer employed by him in his experiments to deter- 

 mine the relative motion of the earth and the ether. He finds satisfac- 



