Photometry of Lights of Dif event Colours. 361 



were so nearly straight as to raise expectations that the law 

 of the flicker photometer had been found. To obtain a more 

 conclusive test, isochromatics were then made. The spectral 

 colour and the standard were matched by the flicker photo- 

 meter at a series of illuminations, stepping down by means 

 of carefully calibrated neutral tint screens. On plotting the 

 logarithms of the slit-widths * against the logarithms of the 

 illumination, the curves of fig. 5 were obtained. These are 



Fig. 5. 



^ Sca,e ffrttnrt 



S//t tv/Ms av te/ws ofya/ue a/, E60l.lt. P/o/fed {oyar/tfim/ca/ty 



Isochromatics, showing variation in relative brightness of spectral 

 colours due to variation of illumination, Flicker Photometer. 



not straight lines. The simple law anticipated does not 

 hold. However, the effect of change of illumination is 

 shown in a manner which indicates the method of study by 

 isochromatics to be an excellent one for following the course 



* For greater certainty slit-widths were not used in this case. Instead 

 the voltage of the test lamp was altered, the intensity change being first 

 determined for each colour by a spectro-photometer in which a variable 

 se ctor disk took the place of the usual variable slit. 



