﻿On the Visibility of Radiation, 301 



We may summarize the above as follows : — Einstein's 

 hypothesis leads necessarily to formula (a) for the entropy 

 and thus necessarily to Wien's radiation-formula, not 

 Planck's. Planck's formal device (distribution of P energy- 

 elements e over N resonators) cannot he interpreted in the 

 sense of Einstein's light-quanta. 



XXXIV. The Visibility of Radiation. 

 By P. G. Nutting*. 



THE quantitative relation between light and radiation has 

 long been sought by many investigators. Herschel, 

 exploring the spectrum with a thermometer, found that the 

 radiation continued beyond what was visible. The invisible 

 ultra-violet portions of spectra were long ago explored by 

 photography. Langley f , twenty-five years ago, explored 

 the infra-red solar spectrum with his fine wire bolometer, 

 and in the visible spectrum measured the amounts of energy 

 of various wave-lengths required for reading print. Pfluger { 

 and Konig and Dieterici § determined the relative amounts 

 of energy required to just produce a luminous sensation 

 in different parts of the spectrum. Konig || continued his 

 investigations from the threshold of vision up to an intensity 

 of about 500 metre candles. 



About ten years ago it was clearly recognized that in 

 order to define light in terms of the radiation which excites 

 it, an intermediate function, the visibility of radiation, must 

 be formulated and its constants determined for the average 

 normal eye. Groldhammer If, in 1905, partly reduced some 

 of Konig' s data and expressed visibility as a function similar 

 in form to that giving the spectral energy of a perfect 

 radiator. Hertzsprung **, in 1906, took a rough average of 

 all available threshold data and formulated visibility as a 

 logarithmic hyperbola. The author ff, independently of 

 Goldhammer and Hertzsprung, reduced the data of Langley, 

 Pfluger, and Konig, and in 1907 published this, a function 



* Communicated by the Author. 



t S. P. Langley, Am. Journ. Sci. xxxvi. p. 359 (1388). 



X A. Pfluger, Ann. Ph. ix. p. 185 (1902). 



§ Konig and Dieterici, Zs. Psy. Phys. Sinn, iv. p. 241 (1893). 



|| A. Konig, Ges. Abhandlungen. 



% D. A. Goldhammer, Ann. Ph. xvi. p. 621 (1905). 

 ** E. Hertzsprung, Z. Wiss. Phot. iv.p. 43 (1906). 

 tt P. G. Nutting, Phys. Rev. xxiv. p. 202 (1907) : Ball, Bu. St 

 p. 261 (1908). 



