718 VERHOEFF AND BELL. 



kinds. First, by absorption in the ordinary sense, and second, by 

 reflection due to the structure of the eye. For instance, the cornea is 

 somewhat lamellar in structure, built up of successive layers and cells 

 and is therefore, owing to the differences in the index of refraction as 

 the ray traverses the structure, somewhat less transparent than if it 

 were homogeneous. There is also a slight loss of energy at the surface 

 in passing into the aqueous. This is also practically homogeneous, 

 but there is again a slight loss by reflection in passage from the aque- 

 ous to the lens which has a materially higher index of refraction, 

 and is of itself non homogeneous from the standpoint of refraction. 

 Finally there is reflection passing from the lens into the vitreous and 

 the vitreous itself is not without structure, so that in fact the path 

 of rays through it can be traced by the faint diffused illumination due 

 to its lack of homogeneity. It is quite impossible to determine accu- 

 rately these losses, except for the initial loss which occurs by reflection 

 at the surface of the cornea to which we have already drawn attention. 

 These losses are greater for rays of short wave length than for those of 

 long, and perhaps the most that can be said about them numerically 

 is that a total loss is probably of the order of magnitude of 10% for 

 rays of medium wave length. 



The general absorption of the media of the eye has been studied 

 by Aschkinass ^ in connection with his determination of the absorp- 

 tion spectrum of fluid water. He found that the transmission of the 

 media of the eye for radiant energy in general was closely similar 

 to that of water in a layer of equal thickness. The large proportion 

 of water in these media would, of course, suggest a similarity and 

 Aschkinass found the characteristic absorption bands of water in the 

 experiments on the eyes of cattle and some control experiments on 

 the human eye. The only notable discrepancy was in finding a con- 

 siderably higher absorption in the cornea than would be warranted 

 by its water equivalent which Aschkinass describes chiefly to a film 

 forming very rapidly over the surface of the dead cornea. In examin- 

 ing the bearing of these facts on the energy focussed upon the retina 

 in any given case it should be noted that the absorption is chiefly 

 in the infra red. Figure 7 shows the absorption curve of a 5'^'^ layer of 

 water as found by Aschkinass ^. Hence in examining the absorption 

 in any given source of energy it will be found relatively greatest for 

 infra red radiation, except for the effect of the lens in cutting off a 

 large part of the ultra violet. Luckiesh ^^'^ has made a study of the 

 absorption of energy from various sources by the eye, based on Asch- 

 kinass's results. From this it appears that from low temperature 



