366 RECORD OF SCIENCE FOR 1886. 



isfactorily solved, the first with the result that achromatic lenses of a 

 much more perfect kiud than has ever been attainable have been pro- 

 duced, and as the outcome of the second a whole series of new glasses 

 of graduated properties have been introduced into practical optics. 

 Associating themselves now with the eminent microscopists Zeiss 

 Brothers, new microscopic objectives have been made of the new glass 

 to which Abbe gives the name apochromatic objectives. These lenses 

 dry give better definition than the ordinary achromatic water immer- 

 sion lens, higher eye pieces may be used with them, and their visual 

 and photographic foci are identical. The new glass is now produced 

 in Jena on the large scale. (Nature, October, 1886, xxxiv, 622.) 



Vogel has determined the variation of refraction with temperature 

 both in glass and iceland-spar. The measurements were made by the 

 method of minimum deviation, and the temperatures were comprised 

 between 12° and 260°, the three hj'drogeu lines and the sodium line 

 being used as points of reference. For 12° the first coefificient for the 

 line D was for white glass 123x10", and for heavy glass 190x10", the 

 second being 106 x 10^^ and 147 x 10". For calc spar the coefficient was 

 for D, 81x10" for the ordinary, and 1020x10" for the extraordinary 

 ray. (Wied. Ann., xxv, 87; J. Phys., January, 1886, IT, v, 45.) 



Bertrandhas described a new refractometer, constructed especially for 

 the optical study of rocks. (J. Phys., May, 1880, 11, v, 223.) 



Gladstone, in a i^aper read at the Aberdeen meeting of the British 

 Association, emphasizes the value of the refraction goniometer in chem- 

 ical work, and suggests its use far more generally than has hitherto been 

 the case. (Nature, February, 1886, xxxiii, 352.) 



Chappuis and Kivi^re have studied the retractive index of air with a 

 view to determine its variation with pressure. They find for their ex- 

 periments at 21° and uj) to a pressure of nineteen atmospheres the fol- 

 lowing formula: n- 1=0.0003554^ (1-1-0.00058 2>), in which n is the in- 

 dex for the line D, and p the pressure in meters of mercury. Hence the 

 index of air for the line D at 0° and 0.76 meter pressure is 1.0002927, 

 identical with Mascart's value. (C. R., June, 1886, cii, 1461.) 



Eayleigh has measured directly the amount of light reflected from 

 glass, as follows : Light from a cloud was passed through ground glass 

 in the window of a darkened room, and made to fall at the polarizing 

 angle on a plate of glass. The transmitted and reflected rays were con- 

 ducted along different paths by a series of reflectors,-but finally emerged 

 side by side and of equal intensity. One of these reflectors was the 

 glass to be tested, the light falling on it at an incidence almost perpen- 

 dicular. This glass was now removed, and a single mirror was shifted 

 so as to make the angles and points of incidence of the reflected ray on 

 the several mirrors the same as before. The reflected ray was now 

 brighter than the transmitted. To reestablish equality a disk with 

 holes in a ring round the center was rotated in the path. The ratio of 

 the sum of the breadths of the boles to the whole circumference of the 



