846 



Mr. H. E. Ives on the 



hue difference the psychological element o£ appraisement is 

 eliminated. As was shown by the last paper, the latter 

 procedure gives results identical to the flicker method. In 

 the present paper, when equality of brightness is mentioned, 

 the straightforward method of comparison as usually carried 

 out with large hue differences will be meant. 



As to the second requirement— the summational one — it is 

 to be borne in mind that while this presupposes the first one 

 fulfilled, it may fail even with that holding. The process of 

 physical summation may introduce factors which are not 

 present or magnify ones not significant in the separate parts. 

 An instance of this is given below. It is to be noted further 

 that certain differences may exist in the results of two 

 methods without any clash with this second requirement. 

 Speaking in terms of spectral luminosity curves, this means 

 that the curves by different methods may differ in the 

 position of their maxima, and may show different directions 

 of change with variation of illumination, but if they maintain 

 their areas the same may all conform to the requirement in 

 question. If, however, the areas of the luminosity curves 

 are different, then the sum of the parts cannot equal the 

 whole by all the methods under test. This is obvious when 

 one imagines the luminosity curve under measurement to be 

 that of a source identical with the comparison standard. If 

 at the end of the spectral measurement the dispersed light is 

 recombined, then, being identical in quality with the com- 

 parison standard, it must measure the same by all methods 

 of measurement. If by one method of measurement this 

 physical summation agrees with the arithmetical summation, 

 derived through the area measurement, it cannot agree with 

 others giving different areas. 



As has been noted from time to time in these papers, 

 curves of different area are obtained by the different methods 

 of coloured light photometry. A striking example of this 

 is given in fig. 1, where are four luminosity curves obtained 

 at one sitting by the methods of visual acuity, critical 

 frequency, equality of brightness and flicker. While they 

 differ little in general shape and position of maxima, the 

 enormous differences in area illustrate forcibly the error 

 fallen into by assuming these methods identical on ihe basis 

 of the similarity in shape of curves obtained by different 

 observers. The most startling curve is the visual acuity one. 

 This was obtained by the use of a test object consisting of a 

 fine line grating placed over the telescope lens. Its area is 

 five times that of its nearest neighbour, which latter differs 

 only by a fractional part from the other two. 



