200 ANNUAL OF SCIENTIFIC DISCOVERY. 



each hand one prism, the two prisms having their thin edges towards each other, 

 we look at the window from the opposite end of the room, we see first two 

 windows with then- vertical lines bent in contrary directions ; but by inclining 

 gradually the optical axes, we can converge them until the two images coa- 

 lesce, and we see only one window: as soon as they coincide the lateral 

 curvature of the vertical lines ceases, and they are bent protectively from back 

 to front : we have then the illusion of a window concave towards the room, 

 such as it would appear reflected by a concave mirror. 



ON THE TRANSMISSION OF THE ACTIVE RAYS OF LIGHT THROUGH 

 THE EYE, AND THEIR RELATION TO THE YELLOW SPOT OF THE 

 RETINA. 



The following is an abstract of a paper, communicated to the Edinburgh 

 Philosophical Journal, on the above subject, by Dr. Wilson. Hartmann, as 

 long ago as 1849, stated, that the eye arrests the chemical radiations which 

 accompany the more refrangible rays of light. Dr. "Wilson not being wholly 

 satisfied with this statement, determined to re-examine the whole subject, 

 and to test conclusively the question, whether the eye can transmit the 

 chemical rays of light. An ox eye was prepared by cutting away the 

 sclerotic coat, until the choroid came into view ; a circular aperture of one- 

 eighth of an inch in diameter was then made through the membranes and 

 the retina, which laid bare the vitreous humor at a point opposite to that 

 where the light enters. The eye was then supported hi the brass mountings 

 of a photographic lens, resting at the posterior end on a ring of cork, which 

 fitted tightly into the tube, and retained in front by a diaphragm, so as to 

 permit the cornea to protrude. From the arrangement of the fittings no 

 light, except that which passed through the eye, could enter through the 

 camera. A collodion glass plate being placed in the box and the camera 

 being properly directed, an image was developed after an exposure of fifteen 

 seconds. Other experiments subsequently made under a variety of condi- 

 tions, such as would seem to obviate every objection, give similar results ; 

 and from these it appears certain, that the chemical or active rays of light are 

 not arrested in their passage across the chamber of the eye. 



An important question to be next considered is, what change do the 

 actinic rays undergo when they fall upon that particular portion of the 

 human retina which anatomists distinguish as the " yellow spot." This 

 " spot," almost peculiar to man, presents a diameter of ~ of an inch, and 

 occupies the bottom of the eye, in the exact axis of its transparent humors. 

 It is more transparent than the rest of the retina, and has long been recog- 

 nised as the seat of the most perfect vision in the eye of man. Now this 

 colored medium of the eye must produce a certain effect on the light which 

 reaches it, and on the actinic rays which traverse it. Prof. Goodsir has 

 shown, that it is not merely the case that light traverses the retina to the 

 choroid, and is then reflected so as to return through the retina, but it is only 

 the rays thus returned which produce a luminous sensation. The light, 

 therefore, which traverses the yellow spot, and is then reflected forwards 

 on the choroidal extremities of the optically sensific constituents of the 



