﻿Visibility of Radiation. 303 



The method of determining the visibility curves was 

 similar to that used by Ives. A wave-length spectroscope 

 was fitted with a Whitman disk flicker photometer, so that 

 the pure spectral hue and a white surface illuminated by a 

 standard lamp were viewed alternately (fig. 1). 



Instead of a glow lamp as a source we used one of the 

 acetylene standard lamps designed by Dr. Mees, and tested 

 and described recently by Mr. Lloyd Jones*. This is 

 essentially a cylindrical flame from a \ foot Bray tip sur- 

 rounded by a metal chimney in which is a re-entrant 

 window screening out all but a horizontal section about 

 5 mm. high. This source is extremely constant in intensity 

 as well as in quality. 



Intensities were varied by means of a pair of Nicol prisms 

 before the slit, the slit remaining of constant width, and 

 therefore the spectrum of constant purity. 



The observing pupil was 0*57x2*57 mm. throughout, the 

 standard intensity 350 mc. or the equivalent of 241 inc. 

 through a pupil of 1 sq. mm. ; test curves run at twice, ^, $ 

 and -jJg- this illumination showed that it was safely outside 

 the range of the Purkinje effect. 



The energies representing equal luminosities were deter- 

 mined by placing at the ocular a Rubens bismuth-silver 

 thermopile connected to a Paschen galvanometer, both 

 made by Dr. Coblentz. This gave the spectral energy 

 distribution of acetylene in the spectrum actually observed. 

 As a further check, the dispersion curve of the spectroscope 

 was determined, and the spectral energy computed from the 

 bolometric data of Coblentz and Stewart on acetylene ; 

 the two determinations agreed throughout. 



The visibility data obtained are summarised in the following 

 tables. Three independent curves were run by each subject 

 on different days. In combining the curves, ordinates were 

 weighted according to height by reducing to equal areas 

 (equal total light in a constant energy spectrum). In fig. 2 

 are plotted individual mean visibilities together with the 

 mean of all 21 subjects. 



The average visibility curve (fig. 3) for the 21 subjects 

 agrees well with that of previous determinations. It is 

 slightly more contracted than that obtained by Ives, the 

 greatest difference from Ives' mean being on the left (blue) 

 side of the curve near the maximum. The mean wave- 

 length of maximum visibility is '555 as against *553 obtained 

 by Ives. 



* L. A. Jones, Trans. I. E. S. ix. p. 716 (1914). 



