and flicker Photometer Speed. Ill 



Plan of Apparatus. 



G — jNlernst glower. 



L — Lens to form image of glower on slit. 

 W — Wheel of coloured glasses to eliminate stray light. 



S — Double bilateral slit. 



C — Wave-length drum of constant deviation spectrometer. 



P — Eight angle prism and transmission diffraction grating. 

 Mi — Mirror reflecting spectral light. 



D — Double image prism. 



N— Rotating Nicol prism. 



A — 1° diameter aperture. 



E — Observing slit, 1/2 X 2 mm. 

 M 2 — Mirror reflecting comparison light. 



O — Opal glass. 



V — Variable neutral tint soreeu. 



B — Comparison light. 



For the reel comparison light a 100 watt tungsten 

 incandescent lamp was used together with a piece of copper 

 ruby glass. This combination was a very close match to the 

 spectrum at *63 /ll. For the blue a 500 watt tungsten lamp 

 in combination with a copper and cobalt blue glass gave a 

 fairly pure blue light. For the yellow-white w 7 as used a 

 1*25 w.p.c. tungsten lamp. For the white w r as used a *65 

 w.p.c. tungsten with " daylight glass/' the latter being the 

 glass developed in this laboratory *, and so chosen for thick- 

 ness that the light transmitted from the lamp was accurately 

 that of a black body at 5000° absolute, as shown by its 

 match with the field of the "Apparatus for the Spectro- 

 scopic Synthesis of Colour," elsewhere described f, when 

 operated with the appropriately calculated disk. 



The experimentally found critical speeds are shown by 

 the heavy points in figs. 2 a, b, c, and d. Owing to the fact 

 that no effort was made to have each case studied at the 

 same field brightness, the actual speeds in the various series 

 have no significance, and they have accordingly been plotted 

 in such units as will most evenly distribute their deviations 

 from the calculated curves. The highest speeds attained 

 were about fifteen cycles per second %. 



* " The Development of Daylight Glass," Brady, Trans. Illuminating 

 Engineering Society, ix, no. 9, 1914, p. 939. 



f " An Apparatus for the Spectroscopic Synthesis of Colour," Ives & 

 Brady, Journal Franklin Institute, July 1914, p. 89. 



X No measurements have as yet been made correlating critical speeds 

 in the polarization flicker photometer with brightness. The speeds are 

 lower for the same brightness than in the abrupt transition flicker 

 photometer. 



