8. P. Langley — Energy and Vision. 



371 



To make clear the way in which we pass from table IV to table 

 V, let us take any particular observation, for instance that 



already cited of April 4th by F. W. V. at 0^55, of 193*8. 

 Referring either to the graphic construction, or to table I, we 



find the value of the tangent (at ^=O"*55)=0-62 approximately, 

 and 193 # 8 X'62=12016. Our table shows the reduction factor 

 for two surfaces of silver to be 1*22, whence the final reduced 

 value becomes 



1-22X120-16 = 146'6 



And in this manner from tables II and III the remaining values 

 in table Y are derived from those in IY ; but here let it be ob- 

 served that these values in table Y do not yet represent what we 

 wish, since they do not correspond in any exact sense to one 

 constant amount of energy. It is true that they might at first 

 sight appear to do so, since one constant quantity of solar en- 

 ergy actually or virtually entered through the same constant 

 width of the slit to produce them, and passed through one con- 

 stant aperture at the second slit, and since finally the prismatic 

 values are reduced to these 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 measures in 

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

 the solar energy in the normal spectrum itself is very unequally 

 distributed. (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 YI 



Sensitiveness of the 



Table VI. 



eye for a constant amount 

 varying wave-length. 



of energy of 



7i= 



0^-34 



0-005,1 

 0000 



0-0026 



0^-38 



0-0168 

 0-0030 



0-0149 



0^-40 



0-104 

 0-139 

 0-140 



0'128 



0^-45 



0042 

 1-50 



3-75 

 2-85 



2-70 



0^-50 



0-194 



7-90 



10-10 



4-73 



7-58 



0^-55 



0-706 

 5-79 

 6-31 

 4-04 



5S8 . 



0^-60 



0-475 

 0-551 

 1-17 

 1-14 



0-954 



0^65 



0073 

 0036 

 0-089 

 0-084 



0-070 



0^-70 



0004 

 0-005 

 0-009 

 0-023 



0-012 



// -75 



0^-768 



S.P.L.. 

 F.W.V. 

 B. E.L.. 

 E. M. __ 



0-00007 

 0-00004 



o-ooooi 



Mean* . 



0-00006 



o-ooooi 



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



