70 



Messrs. H. Hartridge and A. V. Hill. 



Now besides absorption by the eye structures a certain small amount is lost 

 by reflection at the different surfaces and by scattering, since the eye media are 

 not entirely homogeneous. Heat lost by reflection is greatest at the anterior 

 corneal surface, being about 2 - 5 per cent. At the other surfaces about - 5 per 

 cent, is lost. The probable total loss by reflection and scattering we have 

 assumed to be 5 per cent. The values in the above Tables are shown plotted 

 in figs. 6 and 7. Examination of these Tables shows that the heat radiation 

 from \ 11,000 to A, 7000 passes into the eye almost unchecked and a great 

 deal of it reaches the retina. This entirely confirms the results obtained by 

 Vogt mentioned above. • 



Now, we found the iris of the ox totally obstructed heat radiation of every 

 wave-length which fell upon it. It therefore absorbs the same percentage 

 radiation as that which reaches the anterior surface of the lens ; that is 

 roughly 75 per cent, of the heat radiation between \ 13,000 and the visible 

 spectrum. The lens, on the other hand, absorbs of the radiation allowed to 

 reach it through the aperture in the iris only a very small percentage of the 

 incident light energy, approximately 12 per cent. Thus, in the case of the 

 ox and the radiation from a naked Nernst filament, four times the amount of 

 energy is absorbed per unit area by the iris as is absorbed by the lens. The 

 difference is, of course, still greater when unit volume is considered. Now, 

 although an actual coagulation of the lens proteins brought about in the 

 course of time by this small amount of heat radiation is not impossible, when 

 the conclusions of Chick and Martin* with regard to the physical chemistry of 

 coagulation are considered, yet we think it more likely that the change is due 

 to some interference with the nutrition of the lens caused in some way by 

 the enormous heat-absorbing power of the iris affecting the secretion of the 

 aqueous humour by the ciliary body, as Parsons suggests, f It would be 

 premature to speculate what the connection between the heat stimulus on 

 the iris and the secretion of aqueous may be, but several interesting points 

 may, perhaps, be briefly mentioned. 



Firstly, the heat radiation is probably absorbed but slightly by the pigment 

 in the substance of the iris, by far the greater amount of energy passing 

 through and being finally absorbed by the pigment on its posterior surface. 

 In the case of blue-eyed individuals the pigment in the stroma of the iris is 

 absent and the posterior pigmentary layer is alone effective in absorbing- 

 radiant energy. This means that not only does the absorbent layer come in 

 intimate contact with the posterior chamber of the eye, but also with the 



* ' Journ. Physiol.,' vol. 40, p. 404 (1910). 



t ' Affections of the Eye produced by Undue Exposure to Light,' Eeport to Section of 

 Ophthalmology, British Medical Congress. 



