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



the red than to the violet. The retina of this eye is somewhat 

 deficient in black pigment. 



B. E. L., near-sighted, using glasses whose negative focus 

 is 42 centim. 



E. M., a boy of fifteen whose sight is perfect as far as 

 known. 



It will be remembered that throughout this table from 0'*'40 

 to 0'^*70 the light enters through a slit whose aperture is con- 

 stant. If under these conditions the logarithm table can be 

 just read when the slider is one metre from the second slit 

 (sg), the light would be represented by unity ; if at two 

 metres, by 4 ; if at three metres, by 9 ; and so on. As, 

 however, we have already explained, the length of the rod 

 being limited to but little over three metres, for the higher 

 values we are obliged to introduce the photometer-w^heel. 

 For instance, the strongest light observed by F. W. V. was 

 in the prismatic yellow-green corresponding to a wave-length 

 of 0'^*55, where 193'8 was noted. Had the rod been really 

 indefinitely prolongable, the slider would have needed to have 

 been removed to the length of nearly 14 metres. To avoid 

 this the photometer-wheel was interposed, reducing the light 

 to 2V ^^^ ^^6 actual distance of the slider from the slit [s^ 



was, as we may easily see, a / — ^r-r- or 3"11 metres. The 



feeblest light which has been here measured with the standard 

 slit is that by F. W. V. on Aju-il 2nd at wave-length 0''-40, 

 which is put down at "13, corresponding to a distance of 

 36 centim. from slit {s<^. 



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. atO'^-55 of 193-8. 

 Eeferring either to the graphic construction, or to Table L, 

 we find the value of the tangent (at X=;0'^"55)=: 0*62 approxi- 

 mately, and 193-8 X •62 = 120-16. 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 remain- 

 ing values in Table V. are derived from those in IV. ; but 

 here let it be observed that these values in Table V. 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 



