Photometry of Lights of 1) Iff event Colours. 749 



to make a sharp edge, was placed directly behind and 

 parallel to the rotating disk, and so arranged that it could 

 he drawn tip out of place. The second spectrometer was 

 then so placed that the light from its prism was received 

 by the mirror and reflected into the telescope of the original 

 instrument. In this way half the field received light from 

 each instrument, and any two spectral colours could be 

 compared by the equality of brightness method. The flicker 

 disk and comparison lamp being undisturbed, it was pos-ible 

 at any time to secure a flicker photometer curve. As these 

 were proved to be independent of the kind of comparison 

 lamp usedj it was not necessary to arrange to secure such 

 curves for different spectral comparison colours, as might at 

 first sight have been thought necessary for completeness. 



These experiments Were carried out only at the high 

 illumination for the small field size, and with a bright 

 surrounding field, these being the conditions dictated by all 

 the work Up to the present as best for accuracv and freedom 

 from disturbances of all sorts, and so best fitted to be estab- 

 lished as standard. Starting at the middle of the spectrum, 

 the two sides of the field Were made alike in hue and closely 

 the same in brightness, and their relative brightness measured, 

 then one side Was changed to a wave-length only slightly 

 removed and its brightness was measured, the Wave-length 

 difference being such as to introduce only a very slight hue 

 difference. The comparison field was again made like the 

 test side, the latter moved to a new wave-length and the 

 brightness of this step compared With the first, and so on 

 through the spectrum. Then starting at either end with the 

 slit-width values obtained by working from the centre, the 

 spectrum was measured continuously to the other end. 



The curves obtained by this procedure were unsatisfactory, 

 because of cumulative errors. Upon returning from red to 

 blue, after working from blue to red, the original value for the 

 end of the spectrum w r as not obtained. In order to eliminate 

 all possibility of systematic drift such as might be caused by 

 favouring the new hue, or one side of the photometer field, the 

 following scheme was substituted for the first : — At numerous 

 points in the spectrum, at the approximate slit-widths found 

 from the mean of the previous work as necessary for equal 

 brightness, the ratio of brightness of X to X — A\ was 

 measured. For A\ was chosen '004//,, which means every^ 

 where a very small hue difference. The measurements 

 were made at points taken absolutely haphazard, and each 

 measurement consisted of four, first with both sides of the 

 field illuminated bv \ : second, the right-hand side by A. 



