Dr. S. P. Thompson on Photometry. 123 



inequalities on either side of the final position was used ; 

 and indeed it constitutes the basis of the method of measuring 

 the illuminating effect of any coloured light at different parts 

 of the spectrum. In this case also the motion was produced 

 by hand, by shifting a lever rapidly to and fro. 



Indeed it seems probable that, without any specific recog- 

 nition of its meaning or importance, the method of roughly 

 producing periodic variations in the relative intensities of the 

 two lights has been quite commonly used by many individuals 

 in their work with photometers. 



It has seemed to the author worth while to generalize this 

 principle, and to give definition to it, as a recognized prin- 

 ciple in photometry. He has therefore essayed to construct 

 a Vibration-photometer in which periodic changes in the 

 illumination are deliberately produced, in order thereby to 

 effect systematically that which has hitherto been done un- 

 systematically and by hand. 



There are several ways of arriving at the desired result : — 



(1) The " screen " of the photometer may be so mounted 

 on its carriage as itself to vibrate through a small 

 distance, at some convenient frequency. 



(2) The standard light may be moved in some periodic 

 way to and fro through a small distance ; though in 

 the case of flame-lamps this is inadvisable. 



(3) The light of the standard lamp may be made to vary 

 by a small percentage in a periodic manner; one way 

 being by revolving in front of it a fan with narrow- 

 arms to obscure periodically a portion of its light; 

 another way being to vary periodically the aperture of 

 a Metrrven slit. 



(4) The light that is to be measured may be made to vary 

 periodically by some known fraction of itself in ways 

 analogous to the preceding. 



When any of these things are done the position of balance 

 can be found by simple direct adjustment without the usual 

 delays. 



The particular form in which the author has worked out 

 the periodic principle is by mounting upon a spring on the 

 photometer-carriage the paraffin-block translucent " screen " 

 of Dr. Jolly. He finds that a period of J of a second is 

 convenient for the purpose. He has also tried an eight- 

 bladed fan, with narrow blades to obscure in passing a frac- 

 tion of the light. In using with either of these periodic 

 devices the paraffin-block apparatus of Jolly, there is found a 

 curious optical illusion which assists the judgment. As the 

 brightness of the illumination of the two adjacent halves of the 



