426 Mr. J. Gorham. The Pupil- Photometer. [Nov. 27, 



pupil will be found to remain stationary at the same magnitude as 

 before. These results are in strict accordance with the rule that the 

 intensity of the illumination of any body, in the presence of a source 

 of light, will depend upon its distance from that source, and obeys 

 the general law of radiant forces, the intensity of the light varying 

 inversely as the square of the distance of the luminous body. Hence, if 

 a single candle illuminates a body at one foot, four candles at two 

 feet are required to produce equal illumination. 



It will be observed that the process of equalizing the illumination 

 becomes at the same time the measure of the number of candles used 

 in the experiment. Hence, by an extension of the same principle, the 

 candle-power of any luminous body may be found. Thus, if one 

 candle at one foot is equal in intensity to four candles at two feet, 

 it will be similarly equal to nine candles at three feet ; where the 

 number of candles in each case is found by taking the square of the 

 distance. Let it be required, for instance, to take the candle-power 

 of a gas-flame. Suppose now that the distance required to equalize 

 the illuminations ia equal to six feet, then the square of six being 

 equal to thirty-six, the brilliancy of the flame will be equal to that of 

 thirty-six candles. 



The fact of the pupil varying in different individuals under a light 

 of the same intensity, scarcely militates against the use of the instru- 

 ment, for so far as observations extend it has been found that the 

 pupil in each person has a range of its own, and that while the 

 absolute numbers vary the ratios are the same. (See Diagram in 

 paper.) 



In taking observations we are not conscious of exercising any 

 voluntary efforts of our own with a view to controlling the pupil. 

 Its movements, on the contrary, are involuntary and instinctive. 

 Herein, doubtless, consists the certainty with which they are per- 

 formed. 



