154 Mr. H. E. Ives on the 



The difficulty is largely psychological. A decision must bo 

 formed as to when a certain quality of the two illuminations 

 is equal, while other inseparable qualities are different. "With 

 a large colour difference, this decision is difficult ; at first 

 attempt almost impossible. At best, one's settings are apt to 

 vary over a wide range; at another time a different decision 

 may be made, and the average of a set of readings diverge 

 considerably from the set previously made. With small colour 

 differences an observer will with practice make quite definite 

 settings, and his criterion of equality then probably assumes 

 a fairly constant value. As a method of heterochromatic 

 photometry, the method of direct comparison is principally 

 open to objection because of its lack of definiteness of 

 setting. 



As to the second requirement, that the measured light 

 quantities may be treated as physical quantities, i. e., com- 

 pared in any order or added to produce their arithmetical 

 Mini, Abney with his colour-patch apparatus has measured 

 the intensities of several spectral colours separately and then 

 together, finding practically perfect agreement between the 

 sum of the intensities and the intensity of the sum. In view 

 of the difficulty of making exact settings with coloured fields, 

 those results show remarkable agreement, it is to be regretted, 

 however, that these measurements were made with such an 

 inexact instrument as a shadow photometer, that the illumi- 

 nation from the different colours (an arc spectrum) was very 

 different, and that the measurements were confined to very 

 small fields : in short, that they were made under conditions 

 quite different from ordinary photometry. 



As to the method of visual acuity, its chief defect is 

 extreme lack of definiteness. Properly it should be used 

 only for determining different orders of illumination. Visual 

 acuity varies as the logarithm of the illumination and very 

 slowly. For instance, with the best test objects probably a 

 5 per cent difference in visual acuity represents the limit 

 of perceptible difference. Study of Koenig's data on the 

 relation between visual acuity and illumination shows that 

 this corresponds in the middle region of illumination studied 

 to about 30 per cent, difference in illumination. As a method 

 of measuring illumination, visual acuity is therefore almost 

 beneath notice. This is borne out by the work of all 

 observers; so that while visual acuity may be a correct 

 measure of good seeing conditions, it is not a method of 

 photometry. Until, therefore, it shall be proved that the 

 criterion of good seeing, as so determined is very different 

 from that given by other .methods.,. and much more important, 



