610 SCIENTIFIC RECORD FOR 1885. 



the image iu his experiments was ill defined, and criticising in several 

 particulars the new method. {Nature^ May, 1885, xxxii, 6.) 



Some years ago Cornu experimentally established the fact that in 

 quartz the mean speed of propagation of the inverse circular waves along 

 the optic axis is sensibly equal to the velocity of the ordinary wave 

 perpendicular to this axis. Exner has now shown that a more general 

 law can be deduced from the formulas given by Cauchy and Von Lang 

 to represent the properties of quartz, as follows: For any direction 

 whatever the arithmetical mean of the two velocities of propagation is 

 equal to the arithmetical mean of the velocities which would correspond 

 to this same direction if the medium did not possess rotatory power. 

 Von Lang's recent measures confirm this law completely. ( Wied. Ann.^ 

 XXV, 141 ; J. Phys., October, 1885, II, iv," 468.) 



Langley has determined i)hotometricaiiy the amount of light trans- 

 mitted by wire gauze screens, such as are sometimes used to diminish 

 the apparant brightness of stars in making meridian observations. He 

 found that one screen transmitted 0.395, two screens 0-144, and three 

 screens 0-052 of the incident light. In fact, however, the three screens 

 as used with the telescope allowed a transmission of only 0-0014. On 

 investigation this result was found to be due to diffraction. Hence the 

 author concludes, first, that the transmission as measured by the pho- 

 tometer box was equal to the ratio of the sum of the areas of the aper- 

 tures of the screen to its total area and, therefore, could be considered to 

 be the true transmission of the screen ; second, that the much smaller 

 transmission of the screen when used in front of the object glass of a 

 telescope to diminish the apparent brightness of a star is satisfactorily 

 accounted for by the loss of light caused by diffraction under these cir- 

 cumstances ; and, third, that screens used for this purpose should have 

 their constants determined by special experiments of the nature of those 

 now described and that their photometric use should then be limited 

 to the reduction of the light of bodies possessing a small angular mag- 

 nitude. {Am. J. Sci., September, 1885, III, xxx, 210.) 



Fol and Sarasin have investigated the depth to which light pene- 

 trates the Mediterranean Sea. By means of photographic plates they 

 have proved that in the middle of a sunny day in March the rays of the 

 sun do not penetrate below 400 meters from the surface. At 380 meters 

 shortly before 11 A. M., the impression on the plate was less than that 

 which would have been left on exposure to the air on a clear night 

 without a moon. In the lake of Geneva the extreme winter limit is 200 

 meters; but they find as much light at 380 meters in the Mediterranean 

 as at 192 meters in the lake. The light penetrates 20 to 30 meters 

 deeper in the lake in March than in September. (Phil. Mag., January, 

 1885, V, XIX, 70; Nature, June, 1885, xxxii, 132.) 



2. Reflection and Bcfraction. 



Amagat has described an instrument for measuring angles which re 

 ticmbles the sextantj but which has the axis of the telcvscope perpendic- 



