Hutchins — New Method of Measuring Light Efficiency. 529 



Aet. XLIX. — A New Method of Measuring Light Efficiency; 

 by C. C. Hutchins. 



The measurement of light efficiency has been attended with 

 unusual difficulties considering that what is required is merely 

 the ratio of two numbers representing respectively the values 

 of the visible and total radiation of the source. Nor do the 

 difficulties arise merely from the necessity of comparing large 

 things with small, but rather for the want of some method of 

 properly separating the quantities to be compared. 



To obtain a complete energy curve, and to compare the 

 integral of the visible portion with the total is an o extremely 

 tedious and somewhat uncertain procedure. Angstrom's 



method requires an elaborate apparatus and the application of 

 several troublesome corrections. 



It is believed that the following method will be found as 

 accurate, and much more simple and direct than those hitherto 

 in use. 



Theoretical. — Let the figure represent the energy curve of 

 the light source ; the vertical line separating the visible (a), 

 from the infra red (b). If now we define efficiency as the ratio 

 of the visible to the total radiation, w r e have : 



efficiency = 



a-bb 



Let now a water cell a few millimeters thick be placed in 

 the path of the light. Assuming for the moment that the 

 water is perfectly transparent to light ; a -be becomes the trans- 

 mitted energy, the curve coming down to zero at wave length 

 1-8/x. 



