328 SUMMARY OF CURRENT RESEARCHES RELATINQ TO 



Signal-green glass is, so far as the author knows, only made as 

 " pot-metal " and never flashed. The colouring matter is believed to 

 be (di-valcnt) copper. 



Madan's Modification of Foucault's and Ahrens's Polarizing 

 Prisms.* — H. G. Madan suggests that if a film of air (as in Foucault's 

 prism), instead of a film of Canada balsam (as in Ahrens's prism), is 

 placed between the middle spar-prism and the next, the ordinary ray 

 will be totally reflected, while the extraordinary will still emerge and 

 be available as a plane-polarized ray for experiments, as in Foucault's 

 prism. 



This extraordinary ray, however, is not only deviated on emergence, 



but also over-corrected for colour ; but both the deviation and the 



dispersion can be almost entirely corrected by passing the ray through 



a prism of crown glass combined with a 



^'"^•- ^^- prism of very dense flint glass, as shown 



in fig. 80 (a and b calc spar, c crown glass, d 



dense flint glass). The combination forms 



a polarizing prism with an angular field of 



28°, about ec[ual to that of an ordinary 



Nicol's prism, and far greater than that of 



a Foucault's prism (which is only 8°). 



The following points, among others, appear noteworthy in the 



above prism : — 



(1) Its length is scarcely more than twice its breadth, the proportion 

 between the two dimensions being rather greater than in Foucault's 

 prism, about the same as in Ahrens's prism, and much less than in 

 Nicol's prism. (2) Only half the prism is made of Iceland spar, a 

 material which is becoming deplorably scarce and expensive. (3) The 

 combination is not quite free from distortion and chromatic aberration, 

 but this is not serious enough to interfere with its use for many 

 optical purposes, especially as a polarizer. (4) In using it, a dia- 

 phragm should be placed in such a position as to limit the entering 

 cone of rays to 28^, since at a greater angle the ordinary rays are not 

 separated by total reflection. 



" Ahi'cns's polarizing prism is certainly," Mr. Madan adds, " a 

 remarkable one. I do not think that a double-image prism has ever 

 been previously constructed in which the extraordinary ray emerges 

 without deviation, while the other ray is deviated to the extent of 

 very nearly 60°." 



Illumination of Microscopes and Balances.t— In measurements 

 and weighings where high scientific accuracy is needed it is some- 

 times necessary to use artificial means of illumination, and it is found 

 that when reflected light cannot be conveniently introduced, the heat 

 from ordinary lamps causes variations of the temperature of the room, 

 &c., which slightly affect the accuracy of the results to be obtained. 

 By using, however, an incandescent electric lamp fitted inside a glass 

 vessel of water, the light may be even brought near to the Microscope 



* Nature, xxxi. (1S85) pp. 371-2. 

 t Ibid., p. 440. 



