530 Hutchitu — New Method of Measuring Light Efficiency. 



Further let a cell identical with the water cell, but contain- 

 ing a water solution opaque to light but transmitting more or 

 less infra red, be interposed : and let the transmitted ener'gy 

 be d. The transmission curve d must lie within the same nar- 

 row limits with c and the two areas be strictly comparable 

 under like conditions. 



If now we can measure a-\-h, ai-c, d y and the ratio of c to d 

 we have all that is required. For let 



a 



+ 



b 



= 



n 



a 



+ 

 c 

 d 



c 



- r 



m 



a 







m- 



-dr 



then 



a + o n 



Apparatus and Methods. — A thermopile consisting of a 

 single junction at the focus of a small concave mirror set in a 

 proper tube with diaphragms is pointed at the light and expo- 

 sure made by moving a double wooden shutter. The deflec- 

 tion of the galvanometer is reduced by a known amount by 

 putting resistance in the circuit. The deflection so obtained 

 is proportional to n. 



A water cell about 5 mm thick, having thin plate-glass walls, 

 is then placed behind the shutter ; the resistance is removed 

 from the circuit, exposures made as before, and we obtain 

 deflections proportional to m. 



For obtaining d, a preliminary study of a large number of 

 substances showed potassium permanganate to be quite suit- 

 able. It is very strong in color in solution, may be made 

 opaque to sunlight in a thin layer, while transmitting a con- 

 siderable amount of infra red. A duplicate of the water cell 

 is tilled with a solution of permanganate of such strength as 

 to just show the light under investigation, when held between 

 the light and the eye. 



This cell being substituted for the water cell, we obtain a 

 deflection proportional to d. 



To find the ratio r. 



A spectroscope was given the following additions. A long 

 slit was put in the place of the ordinary short one. The eye- 

 piece was removed and replaced by an ebonite screen having 

 a rectangular opening. One edge of the opening was cut to 

 fit the curve of the spectrum lines in the red. The spectrum 

 of the light source is focused upon the screen and the tele- 

 scope adjusted so that the visible spectrum is just cut off by 

 the curved edge. The infra red alone now appears in the open- 

 ing. In this position the telescope is firmly clamped. 



