and Flicker Photometer Speed. 109 



distance is known. In fig. 2 speeds have been calculated 



2c 

 on the basis of .— —500 for the distance between red and 



yellow-green, these being the colours for which a value of 

 this order of magnitude was obtained in the previous work 

 quoted. 



It need hardly be pointed out that the final speed calcula- 

 tions are based on altogether too meagre quantitative data 

 to expect the results to be more than qualitative. They do, 

 however, indicate clearly the nature of the wave-length 

 critical speed curve to be expected if the underlying theory 

 is correct. Perhaps the most important point brought out 

 is that the curve in question has no connexion with the 

 luminosity curve of the equal energy spectrum, as suggested 

 by Troland. For while the luminosity curve retains its 

 shape unaltered as the colour of the comparison light is 

 varied, the critical speed curve, if the theory is borne out, 

 can have its minimum anywhere in the spectrum. The 

 luminosity curve which is most closely represented by the 

 speed curve's reciprocal is that of the comparison light, but 

 a moment's consideration of what would happen to the 

 critical speeds against a monochromatic comparison light, 

 which has zero value through a large part of the spectrum, 

 disposes at once of any speculation along this line. 



Experimental. 



The experimental apparatus is shown in plan in fig. 4, 

 which with its key should be clear without detailed descrip- 

 tion. Two features deserve discussion. One is the variable 

 neutral tint screen at V, by means of which the illumination 

 from the comparison lamp L is varied *. The procedure 

 was to slowly change the current through the Nernst glower 

 at the spectrometer slit, if necessary also changing the size 

 of the slit, until the flicker setting as made by movement of 

 the variable neutral tint screen attained a definite value. 

 Speed settings were then made for this brightness, which 

 was thus the same for all parts of the spectrum. The second 

 point is the use of a set of coloured glasses, W, at the slit, 

 each of rather narrow spectral transmission, by means of 

 which scattered light of other wave-lengths may be practic- 

 ally eliminated at any point. 



* Described in paper by Kingsbury, Journal Franklin Institute. 

 August 1915, p. 219. 



