66 



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



essential fact from our point of view, for it means that the radiation reaching 

 the lens must be of shorter wave-length than 2 - 3 /x, and is therefore able to 

 pass readily through ordinary glass. 



We next turned our attention to the second part of our enquiry, namely, 

 to what extent do the various structures of the eye receive and absorb the 

 infra-red radiations. We attacked the question in two ways, partly by direct 

 experiment, and partly by calculation from the measured absorption of a 

 standard thickness of water. The results obtained by the two methods 

 agreed with one another. 



Table III. — Absorption by Water in Percentage of Incident Heat Energy. 







A.U. 



Thickness. 



Readings. 



Mean. 



Log of 



i<ppi r»TOPfl 1 



7,000 



mm. 



p.c. 



p.c. 



p.c. 



p.c. 







30 '6 



102 -5 



97 



98 



101 



99 



"0044 



7,500 





95 



93 



96 



94 -5 



94 '5 



'0246 



8,000 





91 -2 



89-8 



95 



92 



92 



"0362 



8,500 





90 



92-1 



9J -3 



90 -7 



91 



'0410 



9,000 





88 



86 -9 



86 6 



87 -1 



87 



'0605 



9,500 



10 "5 



72 -3 



72 -6 



73 



71 -8 



72 '4 



"1403 



9,750 





67 -5 



67 



67 -3 



67 -4 



67 '3 



'1720 



10,000 





74 



74 



73 -1 



74 -2 



73 "8 



"13] 9 



10,500 



— - 



90 '2 



91 '4 



91 -5 



90 3 



90 '9 



'0414 



11,000 





85 -5 



85 -3 



85 -4 



84 -5 



85 '2 



'0696 



11,500 





42 -2 



43 



42 -6 



43 5 



42 '8 



'3686 



12,000 



— 



30 -7 



31 3 



30 3 



30 -4 



30 -7 



-5129 



12,500 





33 -2 



33 -9 



33 3 



33 -6 



33 -5 



'4750 



12,750 





33 -5 



33 -6 



34 -3 



33 1 



33 -6 



-4737 



13,000 





27 -6 



26 -3 



26 -4 



26 8 



26 -8 



-5719 



13,500 



3 



43-1 



43 



43 -2 



43 -2 



43-1 



-3655 



14,000 



1 



24 



24 -4 



23 -6 



23 -9 



24 



-6198 



14,500 





56 



5 5 



5-36 



5-6 



5-5 



1 -2596 



15,000 





13 -5 



13 3 



13 -45 



13 -5 



13 -4 



-8729 



15,500 





29 -4 



29 -2 



28 -2 



29 



29 



-5376 



16,000 



3 



12 -2 



12 -4 



12 -45 



12 15 



12 S 



-9101 



16,500 





16 5 



16 -3 



16 -45 



16 9 



16-5 



-7825 



17,000 





14 -3 



13 6 



13 -7 



14 1 



13 -9 



-8570 



17,500 





8-35 



8-6 



8-4 



8-3 



8-4 



1 0757 



18,000 



1 



20 -5 



20 -9 



20 -4 



20-65 



20 -6 



"6861 



18,500 





52 



5 1 



4-8 



4 95 



5 



1 -3010 



19,000 









2 





2 



1 -7000 



19,500 









2-5 





2-5 



1 6021 



20,000 









4-5 





4-5 



1 -3470 



20,500 









6 





6 



1 2218 



21,000 









7-5 





7-5 



1 1249 



21,500 









7 





7 



1 -1549 



22,000 









5 





5 



1 -3010 



22,500 









2-5 





2-5 



1 -6021 



23,000 





















23,500 





















24,000 





















The absorption by water at different wave-lengths is given in Table III. The 

 values were obtained by first measuring the deflection of the galvanometer 



