Hutchins — JYeio Method of Measuring Light Efficiency. 531 



Over the eye end of the telescope is now slipped a tube, 

 having at its farther end a concave mirror and thermal junc- 

 tion. The mirror is large enough to receive the entire cone of 

 rays coming through the screen, and condenses them upon the 

 junction at its focus. This arrangement was used because it was 

 at hand ; a thermopile or bolometer, having a suitable receiv- 

 ing surface, placed immediately behind the screen, would be 

 simpler. An image of the light source being thrown upon 

 the slit of the spectroscope, the water and permanganate cells 

 are placed alternately against the objective of the collimator ; 

 a shutter is moved as before, and the attached galvanometer 

 gives deflections proportional to c and d respectively. 



Remarks upon setting the screen. — It has been customary in 

 determinations of light efficiency to assume some definite limit 

 to the visible spectrum, as *76 p. Such a limit is quite useful 

 for purposes of uniformity, that one man's work may be com- 

 pared with, another's, but only roughly corresponds to fact 

 except in the case of sunlight. The limit of the spectrum is 

 not the same for all lights or all eyes. It would therefore seem 

 more logical to set the screen upon or near the limit of the red of 

 the light under examination, although this will commonly give 

 a smaller value for the efficiency than that obtained from *76 jjl 

 as a limit. This procedure seems further justified when we 

 observe how steeply the energy curve rises at the limit of the 

 red ; so that a very slight movement of the screen upwards 

 in the spectrum diminishes the apparent efficiency largely, 

 while the actual light is diminished by an inappreciable 

 amount. The smaller values therefore more nearly represent 

 the practical efficiencies. 



Example of the method. — The following measurements were 

 made upon a portable acetylene lamp in which the gas is gen- 

 erated by automatically feeding granular carbide into water 

 contained in the base of the lamp. The feed being intermit- 

 tent, the light varies considerably but a fair average could be 

 obtained by distributing the readings over a considerable time. 



The resistance of the galvanometer and circuit is 2*203 ohms. 



Through water the mean of 5 deflections was 202*2 milli- 

 meter divisions. Through permanganate 45*0 divisions. Ten 

 ohms were added to the circuit, and the lamp direct gave 

 230*6 div., which when reduced to the -scale of the others 

 becomes 2531. 



With the prismatic apparatus the mean of 20 readings 

 through water with 4 ohms in circuit was 293;7. The resist- 

 ance of the circuit is 2*203, and the deflection without added 

 resistance reduces to 827*0. 



The mean deflection through the permanganate cell was 

 205*8. 



