856 Mr. H. E. Ives on tie 



temperature of 2360° abs. was found for the lamp in question. 

 The distribution calculated for this temperature was in turn 

 corrected for the selective absorption of the prism, ground 

 glass, and lenses, determined spectrophotometrically, as well 

 as for the dispersion of the prism. Applying the final value 

 of the energy distribution to the readings, the results are 

 obtained for an equal energy normal spectrum. 



Elimination of Artificial Pupil. — In all the previous work 

 an artificial pupil was used and the results were given in 

 terms of metre-candles illumination as viewed through this 

 1 sq. mm. aperture. In working with a spectrometer the 

 use of a small eye-slit is practically imperative. But in 

 practical photometry an artificial pupil of this size would 

 necessitate working at illuminations too high to be prac- 

 ticable with present illuminants if one had to attain the 

 retinal illumination called for by the investigation here 

 described. Were the pupils of all observers of the same 

 size under the same conditions, a reduction factor might be 

 obtained so that the luminosity curve could be found with 

 the artificial pupil and used for a corresponding illumination 

 with the natural pupil. Such, however, is not the case. 



Fig. 1. 



bed 



- y ►, 



Arrangement for securing the same working illumination 

 for all observers. 



In view of these facts it was considered advisable in the 

 present research to have all curves made for a normal pupil 

 illumination. In order to accomplish this, the arrangement 

 shown in fig. 1 was adopted, a is the diaphragm limiting 

 the photometric field ; b is a small spectacle lens of focal 

 length ab ; c is the eye-slit or artificial pupil ; d is a lens of 



