140 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 

 THE OPACITY OF PHOTOGRAPHIC IMAGES. 



Iii a series of pictures of the sun which have lately been 

 taken by photography, Captain Abney states that he finds 

 the opacity of the image by no means to vary directly as 

 the time of exposure. Having obtained a standard grada- 

 tion of intensity of light by causing a starlike aperture to 

 revolve rapidly around its centre, he determined the relative 

 opacities of the images by comparison with smoke-colored 

 glass wedges, and he finds that the degree of opacity after 

 diminishing very rapidly in the first moments of exposure, 

 then diminishes very much slower, and nearly proportional 

 to the time. The images given on dry places are in general 

 somewhat more opaque than those given by weighty plates, 

 especially during the first moment of the exposure. 7 A, 

 XLVIH.,164. 



ON WAVE SURFACES IX OPTICS. 



Cellerier has in a note on the optical properties of elastic 

 media shown in brief that there is no discord between the 

 laws of double refraction as furnished by observation and 

 by theories based upon molecular movements. It is probable 

 that the ordinary ray, either in the crystals of one axis or in 

 the principal sections of the crystals of two axes, has not al- 

 ways the direction that is ordinarily assigned to it. This de- 

 viation, however, without being discordant from theory, may 

 be so small as to escape observation. Finally, renouncing 

 all mechanical explanation of the phenomena, it is necessary 

 to admit that the direction of the vibrations is parallel to 

 the plane of polarization, and therein is a decided confirma- 

 tion of the theory. There is, in fact, at present not any other 

 new hypothesis necessary in order to show the coincidence 

 between theory and observation. JBibllotJi. Univers., Jan- 

 uary, 1874, 23. 



ELLIPTIC POLARIZATIONS OF LIGHT. 



The nature of the light reflected by potassium permanga- 

 nate has been investigated by Dr. Wiedemann, who has ex- 

 amined the reflected light both by the spectroscope and the 

 polariscope. The crystals of the above-mentioned substance 

 were polished upon ground -glass plates, thus giving clean 



