Prof. S. P. Langley on Energy and Vision. 



15 



constant quantity of solar energy actually or virtually entered 

 through the same co istant width of the slit to produce them, 

 and passed through one constant aperture at the second slit, 

 and since, finally, the prismatic values are reduced to those in 

 the normal spectrum ; but, as the writer has shown, not only 

 by theoretical deductions, from what is observed with the 

 prism, but by very numerous measurements in the normal 

 spectrum from a grating by means of a bolometer, the solar 

 energy in the normal spectrum itself is very unequally distri- 

 buted (see Table I.). 



Since thermal and luminous effects vary proportionately in 

 the same ray, it is to be observed that the values in Table I. 

 furnish for each wave-length a divisor which gives not only 

 the heat but the brightness which would have been observed 

 had the prism dispersed the energy which fell on it in such a 

 way that the same amount of energy fell in one part of the 

 spectrum as in another, and thus we finally obtain the values 

 in Table VI. 



Table VI. — Sensitiveness of the Bye for a constant amount 

 of Energy of varying Wave-length. 



x= 



Of^-Sl 



0'"-38 



0'"-40 



Qi'Ab 



0'"-50 



0'*-55 



0^-60 



0'"-65 



0''-70 



Of^-75 



0(^-768 



S. p. L. . 









0-042 



0-194 



0-706 



0-475 



0073 



0-004 







F. W. V. 



00051 



0-0168 



0-104 



1-50 



7-90 



5-79 



0-551 



0-036 



0-005 



0-00007 



0-00001 



B. E. L. 



0000 



0-0030 



0-139 



3-75 



10-10 



6-31 



1-17 



0-089 



0-009 



0-00004 





E. M. ... 







0-140 



2-85 



4-73 



4-04 



1-14 



0-084 



0023 







Mean*. . . 



1 



00026 



00149 0-128 



2-70 



7-58 



5-38 



0-954 



0-070 



0-012 



0-00008 



0-00001 



It will be observed that no correction has been introduced 

 for selective absorption in the substance of the prism itself, as 

 this is absolutely negligible within the limited range of the 

 spectrum we are discussing. 



This table exhibits the relative effect upon very different 

 eyes of a given amount of energy in the form of radiation of 

 various wave-lengths. 



Quite notable differences exist betw^een the different ob- 

 servers, not only as to the absolute sensitiveness of the eye, 

 but also as to the relative efficiency for different colours. 

 This seems to be, to some extent, a function of the age of the 

 observer, if we may draw any conclusion from so few compa- 



* The observations of S. P. L. are here omitted from the mean. 



