
The Simmance-Abady “Flicker” Photometer. 341 
thus found; instead, we multiply —v’ (in motion harmonically 
periodic) by the electric force corresponding to the displace- 
ment thus found, that is by 47 V? times the displacement, or 
by —4V/« times the velocity (e** being the time factor). 
Thus in applying (33) to an electric problem F’ is to be replaced 
by —w’, and F by —47iV/« .(—du/dt) or —47V?u; when 
this is done in the present case all the results agree. 
9. I would take this opportunity of correcting the following 
error in my former paper * :—>p. 671, line 4 from foot, delete 
‘‘ slowest constituent ”’ and for ‘“‘ small” read “ large.”’ 

XL. The Simmance-Abady “ Flicker’? Photometer. 
By Messrs. Simmance & ABaApy f.. ey 4 Yj 
[Plate XX.] { / 4 
HIS photometer is of the alternating-light type, recently 
called in France “ scintillation-photometer,” but more 
generally known in America and England as “ flicker.” 
A very long experience with all the known variations of 
“ flicker ” photometers, and the construction of many others 
of different patterns, resulted in the design of the present 
instrument, and enabled actual rules to be laid down, which 
when adhered to produce a photometer which is most sensitive 
to degrees of lights of the same colour, and also enable the 
intensities of the most violently contrasted tints to be com- 
pared and balanced. 
These rules are as follows:—The light effects must be in 
juxtaposition without any apparent division line, and must 
move, oscillate, or rotate so that the point of juncture of the 
rays of the two lights passes and returns entirely across the 
vision-field. Any hiatus, or longer exhibition of cone light 
than the other, biases the result. The observation surface or 
surfaces upon which the light rays fall, must. be at exactly 
the same distance from the eye, at exactly the same angle in 
relation to the line of sight, and must be of pure white such 
as is afforded, for example, by a clean chalk, plaster of 
Paris, magnesium carbonate, or barium sulphate ; any tint 
affects the accuracy of the result. The observation surfaces 
must also themselves in turn occupy the field of vision; an 
apparent movement or optical illusion does not afford accurate 
results. 
Sir W. Abney, whose work in colour photometry is so well 
known, has placed on record his opinion that in using the 
* Phil. Mag. Dec. 1903. 
+ Communicated by the Physical Society: read December 11, 1903. 


