Simmance-Abady “Flicker” Photometer. 345 
It is obvious, therefore, ithat one obtained the theoretical 
ratios of the one light screened with the first colour in terms 
of the other light screened with the second colour. Then the 
one colour was compared against the other, and the results 
show how nearly this ratio agrees with the theoretical ratios. 
Various initial powers of lights and various lengths of bars 
were used for these experiments. All the readings on the 
bar were computed according to the law of inverse squares; 
and it was therefore assumed that the absence of “ flicker ” 
or point of equality indicated by the Simmance-Abady photo- 
meter is candle-power of intensity of light. 
The following tests are the mean of three persons’ readings, 
and the three individual readings all made independently 
showed practically no variations. 


| Theoretical Ratio. 




| Power in 5 Actual Reading | 
| Candles. € ) : on Photometer. 
Sianl Bed. @| 156 4nearly | 139 
ee al; , ona! Sane ia Saad 
eee gl ee | | Be G8 
ee an Pp 8 108 | 
eee gl 5'] 1-29 1275 | 
Set eae 1-95 1-9 | 
/ 

To detail briefly the construction of this Simmance-Abady 
hotometer :—It consists of a wheel of a white material 
(Pl. XX. fig. 1) with a specially shaped periphery, which 
wheel is caused to revolve before an eyepiece by means of a 
suitable motor. At right angles to the line of sight and parallel 
with the axis of the revolving wheel are the two lights under- 
going examination, the rays of which fall upon the shaped 
periphery of the wheel, enabling the effect of each light to 
be seen in turn through the eyepiece. 
Pl. XX. fig. 2 shows the four cardinal positions of the 
wheel in sequence, illuminated by two unequal lights. It 
will be seen how the light effects travel across the line of 
vision and alternate as the wheel revolves. 
