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XXXI. Studies in the Photometry of Lights of Different 

 Colours. — II. Spectral Luminosity Curves by the Method 

 of Critical Frequency. By Herbert E. Ives *. 



r pHE first paper of this series f described spectral 

 JL luminosity curves obtained by the equality of bright- 

 ness photometer and the flicker photometer. It was found 

 that these curves change differently in response to variations- 

 in illumination ; the equality of brightness method exhibits- 

 the Purkinje effect, the flicker method an opposite and 

 hitherto unobserved shift. At high illuminations the distri- 

 bution of brightness in the spectrum is closely the same by 

 the two methods, at low illuminations quite different. With 

 the standard of comparison used — the unsaturated yellow of 

 a standard carbon incandescent lamp — the two kinds of 

 luminosity curves are usually of somewhat different areas- 

 at their nearest approach to similarity in shape. The 

 differences between the two kinds of curves and the changes 

 in each due to changed illumination are less with small 

 fields than with large. 



The present paper describes certain spectral luminosity 

 curves obtained with the same apparatus and conditions as- 

 before but by the method of critical frequency (sometimes 

 called "persistence of vision"")}. These were studied 

 primarily with the expectation of obtaining some light on 

 the peculiarities of the flicker method, with which the critical 

 frequency method is presumably closely allied. Certain 

 seeming disagreements in the results of other observers 

 furnished a starting point for the work here described. 

 Stuhr, for instance, found that the method of critical 

 frequency gave results identical to those of the flicker 

 photometer. But Haycraft § obtained, by critical frequency 

 measurements on the spectrum, a pronounced Purkinje shift. 

 Since, according to the writer's experiments, the flicker 

 photometer gives a reversed Purkinje effect, it would follow 

 that this identity of the results of the two methods cannot 



* Communicated by the Author. 



t Phil. Mag-. July 1912, p. 149. 



I For details of each of the four methods of photometry discussed in 

 these papers — equality of brightness, flicker, critical frequency, and visual 

 acuity — reference should be made to the first communication. 



§ Journ. of Physiology, vol. xxi. pp. 126-146. 



