On some Iridescent Films. 751 



It follows from this work that previously obtained spectral 

 luminosity curves, such as those of Koenig and Abney where 

 the equality of brightness method was used, are subject to 

 considerable uncertainties. While in the case of Koenig's 

 curves for different illuminations the amount and direction 

 of the changes constituting the Purkinje effect muy be 

 substantially correct, the absolute values for the energy- 

 luminosity values cannot be considered as sufficiently exact 

 to be taken as normal. In view of the results of this paper 

 it is probable that the mean flicker curve given in the first 

 paper of the series is more entitled to be called a normal 

 luminosity curve than any previously obtained ones. 



The main results of this paper may be summarized as 

 follows : — 



1. "With the equality of brightness method the effect of 

 changing the colour of the comparison field and of substi- 

 tuting light for dark surroundings to the photometric field 

 is to introduce irregular and unsystematic shifts and dis- 

 tortions of the spectral luminosity curves. 



2. With the flicker method, the corresponding changes 

 produce no alterations in the luminosity curves. 



3. The equality of brightness spectral luminosity curve 

 obtained by taking small steps of slight hue difference, agrees 

 at high illuminations, for a small field, with the curve given 

 by the flicker photometer. 



4. It is concluded that the flicker photometer gives, under 

 the specified conditions, the true brightness. 



Physical Laboratoiy, 



National Electric Lamp Assoc, 



Cleveland, Ohio. ' / 



May, 1912. ~T" 



- ■ 



LXXVI. On some Iridescent Films. 

 By Lord Rayleigh, O.M., F.R.S* 



THE experiments now to be described originated in an 

 accidental observation. Some old lantern-plates, from 

 which the gelatine films had been cleaned off a few years 

 before (probably with nitric acid), being required for use, 

 were again placed in dilute nitric acid to ensure cleanliness. 

 From these plates a gas-flame burning over the dish was 

 seen reflected with colour, of which the cause was not 

 obvious. On examination in daylight a dry plate was 



* Communicated by the Author. Read before the British Asso- 

 ciation at Dundee. 



