680 KANSAS CITY REVIEW OF SCIENCE. 



Blair observed the after images of lightning flashes. This experiment can be 

 made in a most striking manner by means of the sun. Observe for a few seconds 

 some point on the sky, say ten to thirty degrees from the sun, then move the 

 eyes quickly to some other point, again holding the eyes still for a few seconds^ 

 and repeat the operation for ten or fifteen points around the sun. Then turn 

 partially around, looking at the blue sky or at a bright cloud, and you will ob- 

 serve the after images as usually described. But if you open and close the eyes 

 rapidly, the after images come out with astonishing distinctness. The field of 

 vision will be filled with images of the sun connected by lines which indicate the 

 paths traced by the solar image in passing from one position to the other upon the 

 retina. The color of the images, also, is very marked, and goes through various 

 changes which depend upon the nature and intensity of the light, upon the 

 brightness of the sky which is observed, and probably upon the observer. These 

 images appear to have a motion in the field of view, but this is merely the motion 

 of the eyes, and the experiment will serve to indicate to the observer how difficult 

 it is to keep the eyes still. 



A partial explanation of the phenomena it is not difficult to give. If any 

 brilliant color, (say a bright cloud through a red glass, or a red paper well illumi- 

 nated by diffuse light,) be observed for some time, the sensation of color becomes 

 gradually feebler. Ordinarily this is not noticed, but by closing one eye and 

 observing with the other it will be brought out very distinctly if the eyes are after 

 a time alternately opened and closed. The red will appear much deeper to the 

 eye which has not been fatigued for red. To an eye fatigued for red a white paper 

 would appear as though red had been stricken out, /. e., it would appear greenish, 

 while a green color would appear more intense. The same holds for the other 

 colors. The part of the retina upon which the image of the sun fell was fatigued 

 for a// colors, and hence the after image on a bright ground appears dark at first. 

 The fatigued part of the retina gradually recovers its sensitiveness, and appears 

 to recover more quickly for some colors than for others ; the color of the after 

 image at any moment being thus the combination of the colors for which that part 

 of the retina has at the moment become sensitive. 



Two works recently pubHshed in the "International Scientific Series," will 

 be found exceedingly interesting and instructive in the study of such phenomena, 

 viz., " Modern Chromatics," by Rood, and " Sight," by Le Conte. Both works 

 are admirable, not only in describing many experiments which any one can make 

 for himself, but in presenting clearly and in a simple manner the general princi- 

 ples which enable one to explain similar phenomena. As the eye is the one 

 optical instrument which we all possess, it is certainly natural that we should all 

 feel an interest in the study of its action and its peculiarities. 



