100 Nichols and FranMin — A Spectro-photometric 



The plan in question is easily adapted to meridian observa- 

 tions, but for these we have numerous plans for correcting 

 personal equation, and the writer may also direct attention 

 to the fact of the existence of a distinct device (this Journal, 

 July, 1877) which practically eliminates the personal error 

 in the very act of a transit observation. It is more elab- 

 orate than the present one, which is so simple that it may 

 be useful even in longitude work with the transit, though its 

 proper field seems to be the observation of sudden events ; but, 

 to whatever purpose it is applied, I beg leave to present it to 

 your attention less for any interest that attaches to the partic- 

 ular mechanism exhibited than as an illustration of a principle 

 which seems to me to have not been employed before, at least 

 in this way, and which I trust may have useful applications. 



Art. XII. — A Spectro-photometric Comparison of Sources 

 of Artificial Illumination ;* by Edward L. Nichols and 

 William S. Franklin. 



[Contributions from the Physical Laboratory of Cornell University, No. 5.] 



In the comparison of the various sources of light available 

 for the purposes of artificial illumination, the question of color, 

 though scarcely less important than that of intensity, has been 

 very generally disregarded. 



The neglect of this factor has doubtless been due in great 

 part to the fact that differences of color are not so easily de- 

 termined as mere variations in candle power. Progress in this 

 domain involves the study of the distribution of energy in the 

 spectrum, and since it is with the visible wave-lengths that we 

 have to do, often under exceedingly small intensities, the 

 superiority of the eye in sensitiveness over any device for the 

 direct measurement of the energy of the rays in question, 

 dictates in many cases the choice of an optical method. 



The experiments to be described in this paper were made in 

 the Physical Laboratory of Cornell University during the sum- 

 mer of 1888. They consisted in the spectro-photometric com- 

 parison of various artificial sources of light, and of daylight, 

 with that emitted by a sixteen candle-power incandescent lamp. 

 The sources of light subjected to measurement were : 



1. a standard candle ; 



2. various petroleum flames ; 



* Paper read at the Cleveland meeting of the American Association for the 



Advancement of Science, August, 1888. 



