158 Mr. H. E. Ives on the 



explanation of: retinal action is not at present of great 

 moment. The facts about retinal structure on winch it is 

 based are, however, to be kept in mind while studying 

 colour photometry, as indicating the most probable critical 

 points in the series of changes which may be rung on 

 conditions. 



The complications introduced by colour blindness will not 

 be a main topic of the present research. The object will be 

 to secure data from normal eyes, on the understanding that 

 only observers of normal vision are equipped for photometry 

 witli lights of different colours. The comparison of various 

 methods with one another, and the investigation of each as 

 to its qualifications as a method of measurement, can be made 

 by one observer, and in fact should be so made first of all, as 

 thereby differences in method are not confused with differ- 

 ences between observers. Some data obtained by several 

 observers of normal vision have however been collected with 

 a view to testing the generality of the phenomena found, and 

 to determine how much variation exists among observers 

 with no marked abnormalities of vision. 



Adaptation and fatigue are probably the physiological 

 factors most difficult to estimate and control. Colour vision 

 is peculiarly dependent on adaptation. When physiologists 

 work on problems of vision they distinguish between the 

 light-adapted eye and the dark-adapted eye, depending on 

 whether the eye works in the light or in comparative 

 darkness. The ability to perceive colour, which is lost on 

 greatly decreasing the illumination, gradually returns as the 

 eye becomes accustomed to the small quantity of light. 

 Fatiguing the eye for one colour makes it more sensitive to 

 others. Fatigue also alters the relative critical frequencies 

 of flicker for different colours ; and the effect is different 

 depending on the character of the fatiguing light. 



Both these disturbing factors must be kept in mind m hen 

 investigating heterochromatic photometry, but their detailed 

 investigation must wait until the more prominent phenomena 

 have been covered. It is to be noted that the effects ot 

 adaptation and fatigue are brought out by extreme con- 

 ditions, such as very high or very low illumination. In 

 ordinary photometry the eye is probably in the condition 

 called by physiologists " light adapted," because of the order 

 of illumination used, and because of the use of auxiliary 

 lights for such things as scales and data sheets. If obser- 

 vation has not been too long continued or if the illumination 

 is not trying, the effects of fatigue may be kept at a minimum. 

 The policy in the present investigation has been to maintain 



