174 Mr. H. E. Ives on the 



curves have reversed their relative positions on the red side, 

 and in the comparative measurements by several observers 

 the curves are differently placed on the red side, it has never 

 happened that the flicker curve has shown more blue or 

 green sensitiveness than the equality o£ brightness. In the 

 writer's case, the two curves have been obtained exactly 

 alike within the errors of measurement on two occasions, at 

 two different high illuminations, with the middle-sized field, 

 but this is as near as the blue part of the flicker curve has 

 ever come to crossing over the other. Whether this would 

 always be the case can only be determined by numerous 

 measurements either extended over long intervals or by 

 numerous observers. 



It is possible that this phenomenon of lower green sensi- 

 tiveness by the flicker method may be intimately connected 

 with the tact that the comparison colour throughout was a 

 yellowish white. Certain experiments not yet completed 

 seem to indicate a dependence of the shape of the 

 luminosity curve by equality of brightness on the colour 

 of the comparison lamp. It has been found, for instance, 

 that under certain conditions a spectrum yellow and a spectrum 

 green may each appear by equality of brightness of the same 

 intensity as a spectrum red, yet not appear equally bright 

 when compared directly with each other. One might think, 

 therefore, that a three-part field could be constructed in 

 which two colours each appeared equally bright with the 

 third, at their edges of contact with it, but not equally 

 bright to each other at their common edge of contact. These 

 phenomena of simultaneous contrast are yet to be investi- 

 gated in connexion with the equality of brightness method, 

 and the question must be settled as to whether any similar 

 effects occur with the flicker photometer. 



Perhaps the most interesting feature in these curves is 

 their area. Casual inspection reveals the fact that the areas 

 of the flicker and equality of brightness curves are frequently 

 not the same. Now if the total light is the sum of the parts, 

 this would mean the total light greater with one photometer 

 than with the other. If the spectrum measured were that 

 of a " 4- watt" lamp exactly like that used as a standard of 

 intensity, it would be possible to recombine the dispersed 

 spectrum and make a homochromatic comparison of the two 

 intensities of standard lamp and measured spectrum. This 

 would of course be the same by both photometers. In other 

 words, the physical summation must measure in this case 

 the same, but by our curves the arithmetical summation is 

 different by the two methods. One method or the other, 



