Photometry of Lights of Different Colours. 745 



contrast. As this limitation o£ conditions also held in the 

 work o£ Koenig, who used wave-length *535 fi as his com- 

 parison light, and o£ Abney who used " white," it is a matter 

 of some interest to learn whether this suspicion is upheld or 

 not. The experiments here described give the answer to the 

 question at issue. In addition, another question is investi- 

 gated, namely, whether an alteration in the surroundings of 

 the photometric field, from dark to light, disturbs one's 

 judgment of equal brightness. 



In the first of these experiments no change was made in 

 the apparatus as previously described except that red and 

 green glasses were used in connexion with a high intensity 

 (400 cp.) source in order to obtain red, green and "white " 

 comparison illuminations of the same brightness. The red 

 light corresponded in hue to '62 /a, the green to '55 ft. With 

 these three colours luminosity curves of the spectrum were 

 made under substantially the conditions of the earlier experi- 

 ments. Two illuminations were used, a high one of 300 

 illumination units, a low one of 10 units ; two field sizes 

 were used, one of 1°'85 diameter, the other 5°x8°; the 

 readings were in terms of slit-widths to give equal brightness. 

 The source measured was a normally operated tungsten lamp, 

 as before, but since the results recorded are entirely com- 

 parative no values are given for its energy distribution, nor 

 are corrections made for prismatic dispersion. 



The results obtained were strikingly different in character 

 by the two photometric methods. By the equality of bright- 

 ness method a series of different luminosity curves were 

 found, a different one for each colour of comparison standard. 

 These curves vary from each other by amounts as great as 

 would be caused by considerable differences in illumination 

 for a single comparison standard. One of the most significant 

 things about these curves is that no system or regularity has 

 been found in their manner of deviating from " normal/' For 

 instance, in fig. 1 (p. 746), where are plotted a set of selected 

 characteristic curves from this series (for high illumination), 

 it will be seen that two made with a green comparison light 

 at different times lie at the two extremes of character. 

 There is not a shift in one direction for a green standard, 

 and in the opposite for a red, but apparently any sort of shift 

 may be expected in changing from one comparison light to 

 another. 



It is a common experience in heterochromatic photometry 

 to form a fairly constant criterion of equal brightness when 

 working with a definite set of colours. It is easy to imagine 

 that such a criterion when formed must have some claim to 



