C. GENERAL PHYSICS. 145 



simply gilds the reflecting surface of the prism, in the cam- 

 era-lucida, with a very thin film of gold, and, with Canada 

 balsam, cements to this surface another similar prism. The 

 reflected image appears of the usual yellow color of gold by 

 reflected light, while the transmitted one is green, a difference 

 that is not only not annoying, but in some cases serviceable. 

 The suggestion of Professor Govi has been adopted by Nachet 

 in the construction of various forms of camera-lucida. 14 (7, 

 CCXIIL, 1874, 447. 



COMPRESSIBILITY OF WATER. 



Mascart has shown that the compressibility of water varies 

 more rapidly than the pressure, as has already been observed 

 for other liquids. The methods used by him to investigate 

 the subject have also induced him to measure the heat evolved 

 by the compression of water; or, rather, the lowering of tem- 

 perature when the pressure is suddenly removed. 4 Z>,VII., 

 593. 



NEW PHOTOMETERS. 



Major Elliot reports that in his visit to the establishment 

 of Messrs. Chance, at Birmingham, the scientific director in 

 charge, Dr. Hopkinson, presented him with a photometer of 

 his own invention for the comparison of lights at a distance. 

 It is very compact, and consists of two Xicol prisms which 

 can be moved relatively to each other in azimuth. A little 

 tube carries the analyzing prism, and a second tube con- 

 tains the polarizing prism. The latter being turned until 

 the observed light is just diminished to the point of invisi- 

 bility, and another light being then observed in the same 

 way, a comparison of the angles gives the relations of the 

 powers of the lights. The French Light-house Commission 

 employ a photometer different from Bunsen's, as ordinarily 

 used in America, in which, instead of keeping both the stand- 

 ard light and the one under test fixed in position, the former 

 is moved until the beams from both, after passing through a 

 slit or opening in the photometer, fall upon a pane of glass 

 which has a ground surface, and which, as viewed on the re- 

 verse side, seems equally illuminated by the two lights. The 

 distances from the photometer are then measured by a tape 

 line, and reference to a calculated table shows at once the 



G 



