268 PHYSICS. 



which the ray is properly bent, these prisms being suitably achroma- 

 tized. Light is thrown into them from a mirror at the end of the train. 

 This traverses the system, illuminates the object by reflection from the 

 last surface, also a, mirror, and then returns over the same i)ath, passing 

 to the eye through an opening in the first mirror. The number of prisms 

 which may be used is limited only b}' the light and the definition. — (Na- 

 ture, xxi, 14, November, 1879.) 



Joseph LeOonte has continued his researches on binocular vision, and 

 has published a pai)er on the laws of ocular motion. The conclusion 

 whicii he reaches is that wlien the eyes move in the same direction, 

 parallel to each other, as in ordinary vision of objects, all their motions 

 are governed by the law of Listing; i. e., they are effected without rota- 

 tion on the optic axes. But when, on the contrary, they move in oppo- 

 site directions, as in strong convergence, then the law of Listing is en- 

 tirely abrogated or overborne and another law reigns in its place. — {A/m. 

 J. Scl, III, XX, 83, August, 1880.) 



Steiuhauser has pointed out a fixed relation between the size and rel- 

 ative i)osition of the two views of a stereoscopic picture, the lenses of 

 the camera with which it is taken, and the optical arrangements of the 

 stereoscope in which it is to be viewed. If these relations are rightly 

 observed the effect of relief will be much more perfectly attained for all 

 parts of the picture. The eye-pie(;es of the stereoscope above the plane 

 of the photographic pictures ought to be made as nearly as may be 

 equal to the focal length of the objective of the photographer's camera, 

 and this again should be about equal to the mean distance of easy 

 vision, or from 10 to 12 inches. Hence he suggests: 1st, that all stereo- 

 photographs should be taken with lenses of the same focal length, say 

 lo^"".; 2d, that all should be made of equal breadth or about TS^'^j 

 and 3d, that the distance between the centers of the objective lenses 

 should be constant. — {Nature, xxi, 117, December, 1879.) 



Obach has made a series of experiments to determine whether the 

 phosphorescent light of Bahnain's paint Avould produce any sensible 

 effect on the conductivity of selenium. For this purpose a Siemens 

 selenium cell was used, which was exposed to a phosphorescent surface 

 20 by SOc'", placed at GO^^^ distance. When exposed to the light reflected 

 from the ceiling of a semi-dark room, the light from the phosphorescent 

 surface reduced the conductivity of the selenium 0.7 per cent.; light 

 from the ceiling of a well-lighted room for 15 minutes, 2.4 per cent.; 

 light of sky at 5 p. m. for several minutes, 4.6 per cent.; light from 

 few inches magnesium ribbon, 5.1 per cent.; sunlight for two minutes, 

 7.8 per cent. The influence of 350 square centimeters of luminous sur- 

 face, excited by diffused daylight, \vas found equal to 0.0014 standard 

 candle or 0.04 standard candle per square meter. — {Nature, xxii, 496, 

 September, 1880.) 



Graham Bell presented to the American Association at its Boston 

 meeting a paper by himself and Tainter on the production and repro- 



