336 PEOCEEDINGS OF THE AMERICAN ACADEMY. 



of the human eye, but the eyes of other vertebrates, and of the inver- 

 tebrates as well, have received a large share of attention. The abil- 

 ity of various eyes to form more or less accurate images of external 

 objects has been for the most part inferred from the application of 

 physical laws to the knowledge of their structure. In the case of man, 

 however, there is less need for inference, since his common experience 

 in seeing is a constant demonstration of the ability of his eyes to form 

 images. Nevertheless, even here certain factors which add to the 

 efficiency of the eye as a visual organ, but are in no way concerned with 

 the actual physical formation of the image on the retina, must be taken 

 into account. For example, judgments based upon experience involv- 

 ing other elements than mere image-formation, such as the mental 

 superposition of the images of the two eyes, giving a stereoscopic 

 effect, the action of the ciliary muscles and other mechanisms for 

 accommodation, and the influence of other senses, especially that of 

 touch. The filling out of the blind spot of the retina by the mind is a 

 good example of the influence of experience upon the interpretation of 

 the actual sense impression. It is not unlikely that the eye of a baby 

 is capable of forming practically as good an image as that of an adult ; 

 but the baby lacks the experience and training of the adult which 

 would enable it to form proper judgments, and consequently it appears 

 to have little if any conception of distances and space relations in 

 general. 



The images formed by other eyes than the human can also in a 

 measure be studied from direct observation. Thus the image of distant 

 objects may be seen upon the retina of the freshly removed eye of an 

 albino rabbit, since the absence of pigment leaves the posterior por- 

 tion of the eyeball semi-transparent, and the relaxed eye is accom- 

 modated to distant vision. Or, the posterior portion of the eyeball of a 

 pigmented eye, such as that of a frog, may be cut away, and by aid of 

 a microscope the image may be projected so that it can be observed 

 directly, or thrown upon a screen. This has likewise been accomplished 

 with the eyes of some invertebrates ; thus Exner ('9i) succeeded in 

 taking a remarkable photograph through the eye of a fire-fly, while 

 Parker ('95) demonstrated empirically that the compound eyes of Astacus 

 form a single image rather than a number of separate images.^ Again, 

 in the eyes of many vertebrates the decolorizing effect of light upon the 

 visual purple, after protracted exposure, may be seen upon the retina, 



1 The multiple photographs obtained through the corneal facets of insect eyes 

 do not represent the rethial image formed by these eyes, since in obtaining such 

 photographs only one portion of the dioptric apparatus is used. 



