and the Determination of White Light. 217 



upon the character of the paper used for its surface, and also 

 upon the kind of light that strikes the screen. The intensity 

 of sunlight is continually varying. Its quality may also vary 

 owing to the selective absorption of the atmosphere due to 

 vapour, smoke, and dust. The same conditions do not hold 

 in different parts of the world, and hence the standard equation 

 for white light must vary with the conditions. 



There would be considerable advantage in making com- 

 parisons if one of the standards of intensity were chosen as a 

 standard white ; for instance, the Carcel lamp, the Methven 

 screen, the pentane burner, or the platinum standard. 

 Maxwell's standard equation for white light will not hold for 

 either of these standard sources for the normal eye. 



Lord Rayleigh, in experimenting upon mixtures of red and 

 green light to form yellow light, used as a source an Argand 

 burner covered with an opal globe. This source may be 

 good where the globes have the same selective absorption, 

 and where the gas burnt at the same rate has the same 

 composition. He, like Maxwell, had to contend with a dark 

 line between the two fields to be compared. He also had to 

 use a separate source for a comparison light. 



Even in Helmholtz's colour-mixing apparatus that had two 

 collimators fitted with Nicol and Rochon's prisms, the light 

 from the collimators meets the large dispersion-prism upon 

 ditferent faces, and emerges parallel upon the opposite sides 

 of one of the angles, so that the instrument has a dark line 

 separating the two fields to be compared. 



The utilization of the principle of the apparatus used in 

 this investigation was suggested by Prof. D. B. Brace. The 

 instrument is constructed for the purpose of comparing mix- 

 tures of any number of components from a single source with 

 the original light, or for making a comparison between the 

 original light and any two mixtures. 



From the light-source A (fig. 1) the light passes through 

 the Donder's double slit B, the achromatic lens 0, the prisms D, 

 the achromatic lens E, to the concave mirror Gr, is reflected 

 back through the lens E, the prisms D, the lens C, to the 

 mirror H, and to the observer at 0. 



The radius of curvature of the mirror is equal to the focal 

 length of the second achromatic lens. It is silvered upon its 

 front face, and it is placed upon a movable arm. This arm 

 moves about an axis directly beneath the lens. 



The width of the adjustable slits F in front of the concave 

 mirror G could be measured to tenths of a millimetre, and by 

 estimation to hundredths of a millimetre. The plane mirror H 

 is about 12 centim. long and 5 centim, wide. It is silvered 



