182 Mr. H. E. Ives on the 



the main question. One of these is that the sensibility and 

 reproducibility of the flicker method are sufficiently greater 

 than those of the other method to recommend its use in all 

 cases where colour differences exist and where the question 

 of absolute intensity values is not of the first importance. 

 In the determination of distribution curves of a source, for 

 example, a flicker photometer would be very convenient. A 

 second conclusion is to be drawn from the comparative data 

 by several observers of normal vision. From their differences 

 of reading it is apparent that differences of colour vision will 

 always be a serious obstacle to uniform results in hetero- 

 chromatic photometry. The only practicable escape from 

 this difficulty is to eliminate the need of making such com- 

 parisons in ordinary photometric practice ; in other words, 

 aim to make all practical photometry the photometry of lights 

 of the same colour. 



Under these conditions it is^a matter of indifference what 

 photometer is used, or whether the observer has normal 

 colour vision. The problem of heterochromatic photometry 

 hence becomes one for the standardizing laboratory, where 

 secondary coloured standards or coloured glasses will be 

 prepared for as many practical cases as possible. The search 

 for a photometric method for coloured lights will in this 

 work be treated from that standpoint. 



Of considerable practical interest is the fact that the flicker 

 method most nearly agrees with the method of equality of 

 brightness at high illuminations. Until the problem is 

 further toward solution it would appear well, whatever 

 method is chosen, to make all heterochromatic comparisons 

 at high illuminations. An idea of the order of magnitude of 

 the illumination here meant may be obtained by noting that 

 all the marked shifts (Purkinje phenomena, &c.) occur below 

 70 I.U. This corresponds to about 1/10 that number of 

 metre candles as viewed by the normal sized pupil, or 

 approximately, seven metre candles. 



Acknow ledgem ent. 



The writer takes pleasure in acknowledging his indebted- 

 ness to Dr. P. W. Cobb, Dr. C. F. Lorenz, and Mr. F. E. 

 Oady for their kind assistance in making certain of the 

 measurements here recorded and used; and especially is he 

 indebted to Mr. Matt Luckiesh, not only for making many 

 of the readings but for the preparation of the numerous 

 drawings and the calculation and reduction of results. 



Physical Laboratory, 



National Electric Lamp Association, 



Cleveland, Ohio, December 1910. 



