Photometry of Lights of Different Colours. 849 



than Tufts, showed a discrepancy in the same direction even 

 with white light- 



The points to which attention should be directed here are, 

 1st, that the conditions o£ illumination and field-size are not 

 stated, nor was the illumination constant. Instead of taking 

 portions of the spectrum of equal brightness, equal slit-widths 

 were taken, with consequent low illumination at the red and 

 blue ends. 2nd, there is no mention of correction for the 

 light lost by absorption and reflexion in the prism. It is 

 hard to imagine that this correction was not made, but the 

 test is inconclusive as long as this is in doubt. 



With regard to the peculiar discrepancy with the deep 

 cobalt glass, this can be explained, qualitatively at least, by 

 the results of paper II. of this series. Bearing in mind that 

 the effect of this screen over the eye is to reduce the illumin- 

 ation excessively, it is quite possible that the spectral blue 

 measurements fell in the region indicated by the arrow in 

 the (slit-width) diagram sketched in fig. 2, in a region where 



Fig. 2. 

 Suggested Explanation of Anomalous Results of Tufts. 



Illumina/wnH^ion Used P 



Loq $ln--MJth 



the law of the flicker photometer changes (see paper II. of 

 this series). The result of this would be the effect found by 

 Tufts. However, these low illumination effects, interesting 

 as they are in connexion w r ith the theory of colour vision, 

 cannot be held of primary interest in practical photometry. 



A repetition of this experiment is here described, in which 

 the undesirable features of Tufts' experiments are eliminated. 

 Because this work is intended primarily for application to 

 practical photometry no repetition is made of the low 

 illumination tests, nor tests on observers of abnormal colour 

 vision. 



