726 William Patten 



surfaces is so aclmirablyadapted for reflecting- liglit, iudicates that they 

 may liave a function sirailar to that of the argentea. This supposition 

 is strengthened by the fact that such stmctuves are often found in noc- 

 turnal Insects. 



The views expressed concerning the function of the argentea were 

 the result of the direct Observation of a second image in the retina of 

 Pecten. I inspected ali the more probable sources in hopes of finding 

 some hint as to the function of the argentea in other animals, but with- 

 out success. Even in the large Physiology of Hermann, in Wieders- 

 heim's Comparative Anatomy of the Vertebrates, and in the Histological 

 text-books, I could find no hint or reference to the subject. After my 

 notes upon the argentea were completed , Prof. Doiirn very kindly 

 called my attention to Leuckart's summary in the Handbuch der 

 Augenheilkunde of Graefe and Saemiscii, p. 218, where he speaks of 

 the argentea as intensifying the light efl'ect, in that it causes the same 

 ray to pass twice through the samc rod. Although there is an agreement 

 between these views, which appear to bave been neglected by later wri- 

 ters, and those expressed above, concerning the ultimate eflfect produc- 

 ed by the argentea, i. e. to intensify the light irritation, there is a 

 radicai diflference in the manner in which we believe such an effect is 

 produced. According to the vìcav elucidated by Leuckart, first ex- 

 pressed by Brücke '52a) and subsequently repeated by Helmholtz 

 (52b) a ray of light once entering a rod could not leave it on account of 

 the different refractive indices of the cortical layer of the rod and the 

 axial portion ; each rod would be acted upon by an incideut and re- 

 flected ray, both follo wing approximately the longitudinal axis of the 

 rod. According to this view, then, sincc the rods are pa- 

 rallel with each other, ali the incident and reflected rays 

 would also be parallel, andneither an incident nor'a re- 

 flected image could be form ed! That such an effect does not ob- 

 tain is evident from the fact that a second image is actually formed by 

 the argentea oi Pecten, proving that the rays of light do no t become pa- 

 rallel owing to the reflection caused by the sheath of the rod. In Pecten, 

 at least, the rods form a perfectly continuous layer; the difference be- 

 tween the refractive index of the axial core and that of the rod sheath 

 is, in ali probability, produced by the differentiating effect of the rea- 

 gents. That these changes are probably artifìcial and post-mortem is 

 shown by examining the fresh retina, or lens, when it Avill be seen that 

 the difference between the refractive indices of the nuclei, celi walls, 

 and protoplasm, is so slight as to produce no disturbing optical effect. 



