NATURAL PHILOSOPHY. 161 



upon which appear a quantity of colored spots -without the slightest 

 trace of design or order in their arrangement ; indeed they remind 

 one more of the -dried-lip colors on a painter's palette than anything 

 else. A cylindrical mirror being placed in the middle of the table, 

 reflects a perfect picture of the elevation of the cross, a composition 

 containing six figures, no less accurate in their drawing than beauti- 

 ful in their coloring. One looks in vain for any method or design in 

 the irregular and shapeless mass of colors smeared on the flat board. 



DIRECT ACTION OF LIGHT UPON THE EYE. 



The experiments of Lambert, Fontana, and Weber having shown 

 that light, through the medium of the retina, and the nervous centers, 

 acts upon the iris, it has been supposed to exert no direct action in 

 contracting the pupil, but up to the present time this effect has been 

 considered as a reflex action. 



M. J. Budge has shown, Comptes Rendus, xxxv. p. 564, that the 

 pupil contracts upon exposing the eye to light after section of the two 

 optic nerves, or one only. If in a frog the trunk of the grand sym- 

 pathetic is cut upon one side, below the ganglion of the pneumogas- 

 tric, a section of the two optic nerves being made at the same time, 

 the pupil contracts in an hour somewhat more upon the side upon 

 which the section of the sympathetic is made ; upon removal to a 

 dark place the pupil which was contracted dilates, and again contracts 

 upon exposure of the eye to light, but the light does not act as 

 promptly upon it as upon the pupil of the other side, where only the 

 optic nerve has been severed, without cutting the grand sympathetic. 

 The results are the same upon cutting off the head of a frog, or re- 

 moving the eyes entirely; in this case the pupil contracts under 

 influence of the light, and dilates when taken into a dark place. 

 This phenomenon may be observed for nearly an hour. 



ON THE STEREOSCOPIC COMBINATION OF COLORS. 



M. Dove's researches have reference chiefly to the stereoscopic 

 combination of colors. In 1841 he showed that the stereoscopic com- 

 bination of the complementary colors of polarized rays produced 

 white light. He now makes use of drawings with colored outlines, 

 the colors being dioptric or catoptric : the former he obtains by mak- 

 ing drawings of white lines upon a black ground, and viewing the 

 stereoscopic combination through a colored glass ; in the second case, 

 the figures are drawn upon white paper in the colors which are 

 intended for combination. 



The projection of a convex pyramid was drawn in red lines upon a 

 white ground, and on the same base the projection of a concave 

 pyramid in blue lines. On a second leaf the corresponding drawings 

 were made in the same colors for the other eye. On viewing these 

 drawings in the stereoscope, each pair combined in the usual manner, 

 we should have a convex red pyramid and a concave blue pyramid, 



