PHYSICS. 365 



the index of refraction of water continues to increase below zero, though 

 the density diminishes. {J. Phys., May, 1881, x, 198.) 



• Long has determined the indices of refraction of eighteen compound 

 ethers of the 0nH2uO2 series, at various temperatures, from 18° to 25° 

 €. As a mean, the increase of the index for 1° G. is .00045. From the 



results, he calculates the specific refractive energy — ^ — and the mole- 

 cular refractive energy M f -^^^^ \ Comparing together the opti- 

 cal constants of the butyrates and isobutyrates, it is found that they are 

 lower in every case in the iso-comi^ounds than the normals. The change 

 in molecular refraction for CH2 is found to be, as a mean, 7.69. From 

 this the atomic refraction of oxygen is obtained, 5.77. {Am. J. Sci., 

 April, 1881, III, xxi, 279.) 



Dufet has studied the variation which takes j)lace in the indices of 

 refraction of gypsum with temperature, and finds that the three princi- 

 pal indices diminish as the temperature increases by quantities relatively 

 considerable, but very unequal, compared with each other. {J. Phys., 

 December, 1881, x, 513.) 



Gladstone has communicated to the Eoyal Society a paper on the re- 

 fraction equivalents'of carbon, hydrogen, oxygen, and nitrogen in organic 

 compounds. The refraction equivalent of carbon, when each of its four 

 bonds is satisfied by some other element, does not exceed 5 ; when one 

 bond is satisfied by carbon and the others by other elements, the value 

 is 5; when three bonds are satisfied by carbon, as in benzene, the value 

 is CO or 6.1 ; and, finally, when all four of its bonds are satisfied by car- 

 bon atoms having the value 6, the carbon atom has its highest equiva- 

 lent, 8.8. Hydrogen has only one refraction equivalent, 1.3. Oxygen 

 has two — 3.4 where it is doubly united to a single atom, but 2.8 where it 

 joins two other atoms. ISTitrogen also has two values, 4.1 in the cyan- 

 ides, and 5.1 in organic bases and amides. {Nature, February, 1881, 

 xxiii, 379.) 



Crova has made a study of the aberrations produced by prisms, and. 

 of their influence upon spectroscopic observations. In the first portion 

 he discusses the conditions necessary to obtain a pure spectrum with a 

 minimum of curvature in its lines. He recommends : 1st, a short slit 

 and short prism ; and, 2d, a collimator of small diameter and of long 

 focus to increase the sharpness of the lines, and a telescope also of long 

 focus to increase the magnification. The second part considers the cy- 

 lindrical aberration of prisms, and the third the influence of the elhptical 

 polarization introduced by reflecting prisms, and its correction. {Ann. 

 Chhn. Phys., April, 1881, V, xxii, 513.) 



Anderson has contrived an apparatus called a prismatic optometer, 

 the object of which is to find experimentally the amount of prismatic 

 power and the distance of the center of the lenses which are required 

 in any individual case to so bend the pencils of rays coming to the eyes 



