760 



ENERGY AND VISION 



were kept in the dark, and as we have seen that the sensitivity is in- 

 creased over 100 per cent by a stay of 25 minutes instead of 8 or 10, 

 they cannot well be compared. Generally, at least, they are of the 

 same order of magnitude for the wave length 0.55 fx. A* stay of 1 

 hour in absolute darkness did not seem to increase the sensitivity 

 beyond these figures. 



It must be pointed out that the figures corresponding to the wave 

 length 0.4 ju are doubtful, as the spectrometer which was used was 

 not fit for the measurements in that part of the spectrum, owing 

 to the gilded mirrors. They are only given as approximations. It 

 must also be borne in mind that these quantities of energy do not 



TABLE v. 

 25 Minutes in the Dark. \ 



J 



8 

 4 



correspond exactly to one pure radiation of wave length, 0.55 n for 

 instance, but to the beam comprised between 0.537 /jl and 0.563 ju, 

 the slit covering a range of 0.026 m- 



White Light. — The same technique was applied to the minimum 

 visihile for white Hght (Nernst lamp), and gave 3.8 X 10"^^ ergs, for 

 continuous impression by total radiation. This figure agrees well 

 with that of Grijns and Noyons for the Hefner lamp, 4.4 X 10"". It 

 is better to compare figures related to total radiation, because these 

 figures do not involve the more or less arbitrary choice of limits of 



integration and the knowledge of the ratio - for the considered 



R 



source. 



