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



of the distance from the slit to the screen. The nearest 

 position of the screen brings it within 20 centim. of the slit, 

 the furthest is over 300, so that we have the power of 



— - ) or over 225 times. This, 



however, is by no means a sufficient range for the comparison 

 of the light in the yellow-green with that in the extreme red; 

 and because the graduated rod was not long enough to thus 

 give the desired range, a photometer-wheel was introduced 

 in some of the measurements between the siderostat mirror 

 and the remote slit [si). This photometer-wheel is capable 

 of reducing the light from '50 to '05 or further, and is more 

 fully described in Memoirs National Academy of Sciences, 

 vol. iii. Memoir on the Temperature of the Moon. We have, 

 then, without altering the slit, a range of adjustment through 



225 

 over — - or over 4500 times. The slit Si where the light 



first enters has doubly moving jaws, controlled by a micro- 

 meter-screw. Its standard opening in these experiments for 

 light comprised between A- = 0'^-40 (violet) and \=0'''65 

 (red) M'as 0"1 millim., but it has been opened for supple- 

 mentary experiments to 5 millim., so that we have by opening 

 or closing it a range of light from 50 to 1. It was, however, 

 constantly kept at the standard opening of 0*1 millim. until 

 the main series of experiments was completed, so as not to 

 vary the light by attempting to reset it by the screw. Ad- 

 mitting, however, that for any given prism, transmitting any 

 given ray, the light is sensibly proportional to the width of 

 the slit (which may vary from 50 to 1), to the disposition of 

 that coming through the photometer-wheel, which may vary 

 from 20 to 1, and to the inverse square of the distance of the 

 slider from slit .Sg (225 to 1), we have a possible range of 

 50x20x225 = 225,000 to 1. This, however, it will be 

 understood, has only been employed in our supplementary 

 measurements. 



In the following table all observations, whether made with 

 or without the photometer-wheel, or with a wide slit, as in 

 the case of the supplementary observations in the most feebly 

 luminous portions at the extremities in the spectrum, have 

 been reduced to these standard conditions : — 



Photometer-wheel absent ; 



Slit {si) 0*1 millim. wide ; 



Slit (sg) 1 millim. wide ; 



Slider with logarithm table at 1 metre from slit Sj- 



