PHYSICS. 369 



violet spectrum by means of observations made at different altitudes. 

 He concludes that if the absorption of the ultra-violet rays was due ex- 

 clusively to the action of the vapor of water distributed with the altitude 

 according to the law which experiment indicates, the increase of visibil- 

 ity of the ultra-violet solar spectrum would be a unit (millionth of a 

 millimeter) on the scale of wave-lengths for every increase of 286.9 

 meters. Direct observation having given three times this value, that 

 is, a unit for 868.2 meters of ascent, the theory must be rejected that 

 vapor of water is the exclusive cause of the absorption of the ultra-violet 

 rays. {J. Plii/s., January, 1881, x, 5.) 



Eayleigh has communicated to the British Association some experi- 

 ments which he has made on color, i^rincipally physiological. After the 

 construction of a new instrument for the examination of compound colors, 

 he discovered an interesting peculiarity of color-vision entirely distinct 

 from color-blindness. The red and green mixture, which to his eyes 

 and to those of most people matches perfectly the homogeneous yellow 

 orf the line D, appeared to his three brothers-in-law hopelessly too red, 

 almost as red as sealing-wax. The proportion of red had to be greatly 

 diminished to suit their eyes, until to normal sight the color was a fair 

 green with scarcely' any approach to yellow. [Nature, No^'ember, 1881, 

 XXV, 64.) 



Dubois has suggested an experiment complementary to that of throw- 

 ing a green image and a red one on a screen, superposed for the purpose 

 of making white. He takes a j)iece of red glass and a similar piece 

 of green, pure and well-selected colors. These are placed together 

 in a frame so that one overlaps the other by one -half its length. There 

 are then four quadrants : one white, where there is nothing, one red 

 one green, and the fourth black, where the overlapping occurs. {J. 

 Fhys., October, 1881, x, 448.) 



Lecher, using a thermo-electric apparatus in connection with a pyr- 

 heliometer, has arrived at the conclusion that the amount of carbonic 

 acid which has been proved to exist in the air is sufficient to cause the 

 absorption which has generally been attributed to aqueous vapor alone. 

 He believes his method is preferable to the ordinary chemical ones for 

 determining the amount of this gas in the air. {Wied. Ann., 1881; II, 

 xii, 466; Am. J. Sci., May, 1881, III, xxi, 401.) 



4. Interference and polarization. 



Lommel has described some simple experiments in interference, which 

 avoid the objections made to the mirrors of Fresnel. The surface of a 

 plane black mirror is covered with India ink, with the exception of two 

 bands 6 millimeters wide, and 15 millimeters apart. If a solar beam 

 from a slit falls on the mirror at an incidence of 85° to 88°, the image 

 received on the screen is channeled with interference bands. The same 

 result may be obtained, of course, with two rectangular mirrors a centi- 

 S. Mis. 109 24 



