312 Mr. W. Le Conte Stevens's Notes 



consequently the mean kinetic energy of internal motion (?'. e. 

 of that relative to the new axes of coordinates), is the same 

 for the atoms. 



2. A gaseous mixture distributes itself in a horizontal tube 

 rotating about a vertical axis, exactly as if each of its consti- 

 tuents were present alone in equilibrium under the action of 

 gravity and of centrifugal force. A tube 1 metre (/) long, 

 with one end in the axis of rotation, must make about ten (n) 

 revolutions per second in order that a mixture of hydrogen 

 and carbon dioxide shall contain at one end 1 per cent, (/>) 

 carbon dioxide more than at the other. The rotation must 

 last about two hours in order that the previous deviations from 

 a stationary distribution shall become about one hundred 

 times smaller ; p is proportional to the square of the velocity 

 of the moving end of the tube, and therefore to Fn 2 . 



XXXV. Notes on Physiological Optics. 

 By W. Le Conte Stevens*. 



IN the ' Philosophical Magazine ' for May 1882 the present 

 writer discussed certain phenomena of vision under vari- 

 able physiological conditions. Among these was stereoscopy, 

 attained from a pair of perfectly similar diagrams, with paral- 

 lelism or slight divergence of visual lines, the binocular re- 

 sultant image being caused to appear concave, convex, or plane 

 at will, by properly adjusting the cards in position so that the 

 two retinal images from them could be made either slightly 

 dissimilar or alike. A geometric discussion of this was given 

 in connexion with the record of other experiments that illus- 

 trated the important effect of muscular action in modifying 

 our unconscious interpretation of retinal sensations. This 

 discussion was preceded by a consideration of the current 

 theory of corresponding retinal points, which was accepted 

 only in a modified sense, and not mathematically. It was 

 assumed that, in examining the binocular resultant, freedom of 

 motion is allowed the eyes — a condition that has usually been 

 found necessary when stereoscopy by this method is performed 

 for the first time by any one who is not skilled in binocular 

 experiments. Even at that date the writer was convinced that 

 play of the eyes was not indispensable, however effective it 

 might be in confirming the visual judgment. The geometric 

 discussion, though correct so far as it extends, was not deemed 

 capable of covering all the facts ; but to test the extent to 



* An Abstract from two Papers read before the American Association 

 for the Advancement of Science at the Montreal meeting in August 1882. 

 Communicated by the Author. 



