PHYSICS. 117 



fibrils of the retina, which are sensitive to red, resume their 

 functions soonest. Hence the author infers that the ap- 

 paratus in the eye for the reception of waves of medium 

 length is more liable to be overstrained than that designed 

 for waves of greater or lesser length. 



Kiihne has made some new and remarkable experiments 

 in optography. Following out the suggestion of Boll, that 

 the retina of an animal kept in the dark for a long time is 

 purplish red, the color being bleached by daylight, he has 

 succeeded in fixing upon the retina the image of objects seen 

 by the animal before death. To repeat the experiment, the 

 animal, after being kept for a long time in the dark, is de- 

 capitated, and each of the eyes exposed in turn to a bright 

 object as, for example, the skylight of the laboratory. 

 The retinae are removed from the eye in presence of sodium 

 light, and placed in a five-per-cent. solution of alum. After 

 becoming hard, they may be separated from the optic nerve 

 and inverted. Upon a beautiful rose-colored field a brilliant 

 and sharply defined image of the skylight appears, showing 

 even the sashes. In one of Kiihne's experiments a second 

 image appeared, to his surprise, but it was due to the second 

 skylight with which the laboratory was lighted. 



Carey Lea has studied the sensitiveness to light of various 

 salts of silver. Premising that these salts are sensitive, 1st, 



CJ 7 1 



by being darkened, 2d, by receiving a latent image rendered 

 visible by a deposit of metallic silver, or, 3d, by receiving an 

 image which is made visible by decomposition by alkalies in 

 connection with reducing agents, he gives the results of his 

 experiments to ascertain the sensitiveness of various salts in 

 the third way above mentioned. Silver platinocyanide gave 

 the strongest image, though none of the substances tried at 

 all approached the haloid silver salts in. sensitiveness. More- 

 over, he observed that no substance insensitive in the absence 

 of tannin became sensitive by its presence. 



4. Interference and Polarization. 



Trannin has devised a new method of photometric meas- 

 urement for lights of different colors, with which he has ex- 

 amined and compared the luminous intensity of different por- 

 tions of the spectrum. The method consists in superposing 

 the spectra of the lights to be compared, and then producing 



