Transmission of Infra-red Rays and of Radiant Energy. 63 



in focus of the condenser did not alter the values at any one point. The values 

 we obtained at different times with different lens preparations agreed with 

 one another. At last the true cause occurred to us, namely, that the lens only 

 contains perhaps 90 per cent, water and we were therefore comparing two 

 unequal thicknesses of water. In making further series we therefore reduced 

 the thickness of water from 10"15 mm. to 8*7 mm. by a thin glass plate, the 

 same being done in examining the vitreous and aqueous humours. 



Table II. — Comparative Values of Amount of Infra-red Energy of Different 

 Wave-length transmitted by the Lens and Aqueous and Vitreous 

 Humours and by an Equivalent Thickness of Water. 



Wave-length 

 in A.TJ. 



Water. 



Deflection of galvanometer. 



Aqueous. 



Vitreous. 



Lens. 







mm. 



mm. 



mm. 



13,500 



14 



13 



15 



22 -8 



13,000 



44 



40 



50 



53 -5 



12,500 



50 -5 



48 



56 



54 -1 



12,000 



41 



38 -5 



44 



44 



11,500 



48 -5 



45 V 



53 



49-5 



11,000 



78 -5 



75 



89 -5 



75 



10,500 



71 -5 



68 -5 



83 



71 



10,000 



51 



50 



59 



49 



9,500 



42 -5 



38 -5 



46 



42-5 



9,000 



44 



40 



47 



38 -5 



8,500 



35 



31 -5 



37 



31 



8,000 



26 -5 



24 



30 



25 



7,500 



18 



16 -5 



21 -5 



15 -5 



This Table is shown plotted in figs. 3, 4 and 5. 



The correspondence between the absorption curves obtained for the different 

 eye media and for water was now nearly complete, the values given in 

 Table II and shown plotted in figs. 3, 4 and 5 may be given as examples. 

 It seems clear therefore that no considerable difference exists between the 

 absorption bands of the eye media and those of water. This conclusion 

 which we had already reached has been confirmed by finding a paper by 

 Aschkinass,* who investigated the permeability of the eye media to red 

 and infra-red rays. 



Aschkinass first made a careful investigation of the absorption bands of 

 water, in thicknesses from 10 ft up to 1 metre ; he found bands at - 77 //,, 

 l'O fi, 1*25 /a, 1"5 //., and T94 fx,. The method used was that of the bolometer, 

 in which one scale-division on the galvanometer represented 30 x 10 -6 °C. 

 Our method must be some six or seven times as sensitive as this, and the 

 * 'Wied. Ann.,' vol. 55, p. 401 (1895). 



