Photometry of Lights of Different Colours. 161 



much less for small fields. That is, by using high illuminations 

 and small fields the differences between the methods are less 

 than for low illuminations and large fields. At once there 

 opens the possibility that by proper choice of illumination and 

 field-size the two methods may agree. Should this be the 

 case, it would be a strong argument for choosing that illumi- 

 nation and field-size as the standard one for making colour 

 comparisons. The possibility thus indicated is sufficient for 

 undertaking a complete comparison of the methods. 



Apparatus and Procedure. 

 The method of attacking the problem was to determine 

 by the two methods and under various conditions the lumi- 

 nosity curves of a spectrum of known energy distribution. 

 The apparatus used is similar in some respects to that of 

 Tufts, and is shown in fig. 1. A Hilger constant-deviation 



Fig. 1. — Arrangement of Apparatus. 



Observing slit. 

 Prism table. 

 Collimator slit. 

 Divided drum. 

 Wave-length drum. 



6. White sector-disk. 



7. Photometer bench. 



8. Standard lamp. 



9. Diffusing glass. 



10. Light source. 



11. Photometer bench. 



12. Diaphragm. 



13. Diaphragms used. 



wave-length spectrometer forms a spectrum of the light 

 source upon an eye-slit (1) of dimensions J by 2 mm. The 

 eye placed at this slit observes the prism face (2) illuminated 

 " Phil. Mag. S. 6. Vol. 24. No 139. July 1912. M 



