716. Dr. R. A. Houstoun on the Relative 



any part of the spectrum in candles/sq.cm. divided by the 

 intensity of radiation in ergs/sq.cm.sec. may be defined as 

 the visibility of that part. To determine the relative visi- 

 bility o£ the different parts of the spectrum we must deter- 

 mine the luminosity curve of the spectrum, and the 

 distribution of energy throughout it. This paper * describes 

 the first determination that has been made of the relative 

 visibility of light of different colours to the average human 

 eye. Previous observers have contented themselves with 

 luminosity curves alone, without specifying the distribution 

 of energy in the spectrum (Sir Wm. Abney), or if all the 

 data have been given, it is only for one observer (Prof. A. 

 Konig's results reduced by P. G. Nutting f). The relative 

 visibility of the different colours of the spectrum is of course 

 important with reference to heterochromatic photometry and 

 the definition of light f. 



The number of people who made complete tests was 52, 

 most of whom were honours students in physics. . The 

 experimental work fell into two parts: (1) the faking of the 

 luminosity curves for each person tested, and (2) the deter- 

 mination of the distribution of energy in the spectrum, which 

 had only to be done once for all. 



A considerable time was spent choosing an apparatus for 

 taking the luminosity curves. Abney J s well-known method 

 was experimented with and rejected, because it is very 

 wasteful of light, and requires an arc or Nernst filament as 

 source, and these sources were not considered constant 

 enough for a long series of tests such as was contemplated. 

 Then a spectroscope was employed with its eyepiece removed 

 and an aperture substituted. The eye of the observer looking 

 in then saw a face of the prism illuminated in monochromatic 

 light ; the lower part of the prism was covered with a white 

 card illuminated from the side, and the intensities of the 

 card and prism were matched by altering the width of the 

 slit. This method was tried also with a Thorp diffraction 

 grating instead of a prism, but was rejected owing to its not 

 being accurate enough. Finally, the following original 

 method was employed and satisfied all requirements : — 



A Hilger constant deviation spectroscope was taken, and 

 two little total reflecting prisms were mounted in the eye- 



* Since this paper was written a series of articles by Mr. H. E. Ives 

 with the same purpose has appeared in the Phil. Mag - . This and the 

 following sentence are consequently not true. 



t Bull. Bureau of Standards, vii. p. 1>38 (1911). 



% R. A. Houstoun, Proc. Roy. Soc. A. lx<cxv. p. 275 (1911). 



