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LXVIL A New Method of Measuring the Luminosity of the 

 Spectrum. By Frank Allen, M.A., Ph.D., Professor 

 of Physics, University of Manitoba, Winnipeg* . 



WHEN a ray o£ light entering the eye is periodically 

 interrupted by a rotating sectored disk, a sensation 

 of: flickering is produced until the interruptions reach a 

 certain critical frequency at which the impressions of the 

 separate flashes of light become fused into one continuous 

 sensation. This peculiarity of vision, in one form or another, 

 was observed and commented on by philosophers in ancient 

 times, but was first quantitatively investigated by the 

 Chevalier D'Arcy, who in 1765 measured the least time a 

 revolving glowing coal required to trace an apparently con- 

 tinuous circle of light. In more recent times this subject 

 has been studied by numerous investigators, and many 

 phenomena of interest and importance elucidated. 



It was discovered by Ferry t, and subsequently, in another 

 manner, by Porter J, that the duration of the sensation of 

 undiminished brightness of a flash of light, at the critical 

 frequency of interruption, depended only on the luminosity 

 of the light and not in any way on the colour. The duration 

 of the impression was found by both investigators to be 

 inversely proportional to the logarithm of the luminosity of 

 the light. 



In the course of some investigations on colour vision in 

 which constant use was made of the measurement of the 

 critical frequency of flicker, it became desirable to measure 

 the luminosity of the spectrum in some direct manner. As 

 apparatus for the more commonly used methods was not 

 available, a method based on the above principle of Ferry 

 and Porter was devised which is believed to possess some new r 

 features. 



The arrangement of apparatus is shown in fig. 1. Light 

 from an acetylene flame (A), after concentration by a lens (B), 

 passed through an open sector of the disk ( D), which was 

 rotated by an electric motor, then through tw 7 o Nicol prisms 

 (E and F) arranged with their principal sections horizontal, 

 thence through the spectrometer (G), and was finally viewed 

 in a Hilger eyepiece (H) in which all the light of the 

 spectrum, except a narrow central band of any desired 

 colour, was cut off by means of adjustable shutters. 



In the path of the light at C a small mirror was set so as 



* Communicated bv the Author. 



t E. S. Ferry, Am. Journ. Sci. vol. xliv., 1892. 



X T. C. Porter, Proc. Roy. Soc. vol. lxiii., 1898 : vol. lxx., 1902. 



