Photometry of Lights of Dif event Colours. 175 



perhaps both, must be deficient as judged by the ability to 

 add or integrate the measured quantities. This question is 

 being investigated further. 



(£) Comparative results obtained by dif event observers. 



When these measurements were finished it was thought 

 well to obtain some measurements by other observers in 

 order to serve as a check, and also to obtain data on dif- 

 ferences to be expected between observers. Five observers 

 in all were available, and these are indicated by initials. 

 H.E.I, and M.L. are the two having previous experience 

 with the apparatus ; P.W.C., no previous experience with 

 apparatus, some experience in photometry, considerable ex- 

 perience in observing with various optical instruments ; 

 F.E.C, long experience in photometry ; C.F.L., considerable 

 experience with optical instruments and some in photometry. 

 These observers have all perfectly normal colour vision. 

 Before these measurements each observer made a number of 

 settings with a three-colour mixing instrument (Ives' colori- 

 meter) to match white. From these settings it appears that 

 the observers differed from the mean by no more than 5 per 

 cent, in the proportions of red, green, and blue necessary to 

 make the match. Differences of this amount are found by 

 the writer to be the rule with all who fall in the class of 

 " normal vision/' Those occasional observers who would be 

 classed as partly colour blind will differ by forty or fifty 

 per cent, from the writer or from the mean of several 

 " normal observers." 



Two illuminations were determined on, a high one of 250 

 units, a low one of 10 units, and the middle size of field was 

 used. This choice of illumination and field-size was made 

 in view of the previous work. The high illumination is one 

 beyond which no changes in the relative positions of the 

 curves are to be expected, the low illumination one at which 

 flicker measurements are still easy to make. The middle 

 size of field was chosen partly because it is the easiest one at 

 which to make observations, and partly because the nearest 

 coincidence of the two kinds of curves had been obtained for 

 this size. 



The apparatus was left undisturbed between experiments. 

 Except in the case of the observers H.E.I, and M.L., the 

 divided head on the slit was read by an assistant. The 

 tungsten lamp was a new one which was given a short 

 seasoning, and at the conclusion of the measurements its 

 energy distribution was immediately measured, as before 

 described. 



