PHYSICS. ]19 



those two sets are due to interference taking place within 

 the film of balsam at the critical angle of total reflection for 

 ordinary and extraordinary rays respectively; they are, 

 therefore, analogous to the interference bands in a thin film 

 placed beneath a prism of a more highly refracting sub- 

 stance, and occurring just within the limit of total internal 

 reflection, as first observed by Sir W. Herschel. 



Stone has exhibited to the London Physical Society some 

 diffraction gratings on glass and metal, ruled by W. Clark, 

 of Windsor Terrace. The majority of them were close spi- 

 rals, about 1000 to the inch, which gave brilliant circular 

 spectra, the slight difference between spirals and true circles 

 not being apparent. The metal gratings were linear, 1000 

 to the inch, the spectra being much more brilliant than the 

 refracted ones. The German silver and cast steel hitherto 

 employed not being suitable, the author proposes the use of 

 speculum metal. The idea is not new, Saxton having ruled 

 lines for this purpose for Bache many years ago. The exqui- 

 site speculum-metal gratings of Rutherford are well known. 

 He much prefers them to glass silvered, and his latest tri- 

 umphs in this direction abundantly justify the preference. 



Andre has studied theoretically the phenomena of diffrac- 

 tion in optical instruments and their influence on astronom- 

 ical observations. 



Bezold has suggested another and a very convenient 

 method for studying the laws of color-mixture. A prism 

 of Iceland spar is placed in the interior of a blackened tube, 

 which is closed below by a disk having four squares cut out 

 of it. The pi-ism^ of course, gives, on looking through it, 

 double images of the squares, and in a certain position two 

 of the eight are brought to coincide with two others in the 

 middle. Surfaces of different colors being brought under the 

 two squares occupying, say, the upper row, their composite 

 color is obtained in the middle image. It is then easy to find 

 what color must be put under the lower two squares to ob- 

 tain a color in the middle corresponding to the one above. 



Baily has examined microscopically the optical properties 

 of starch grains. He concludes that they are transparent 

 bodies, consisting of an interior nucleus surrounded by coats, 

 and explains their appearance in polarized light by suppos- 

 ing the starch to be doubly refracting, with two axes of elas- 



