1887.] on Sunlight Colours. 63 



cast another shadow from a candle or an incandescence lamp, and the 

 two shadows are illumiuated, one by the light of the coloured patch 

 and the other by the light from an incandescence lamp which I am 

 using to-night. [Shown,] Now one stripe is evidently too dark. By 

 an arrangement which I have of altering the resistance interj^osed 

 between the battery and the lamp, I can diminish or increase the light 

 from the lamp, first making the shadow it illuminates too light and 

 then too dark compared with the other shadow which is illuminated 

 by the coloured light. Evidently there is some position in which 

 the shadows are equally luminous. When that point is reached, I can 

 read off the current which is passing through the lamp, and having 

 previously standardised it for each increment of current, I know what 

 amount of light is given out. This value of the incandescence lamp 

 I can use as an ordinate to a curve, the scale number which marks the 

 position of the colour in the spectrum being the abscissa. This can 

 be done for each part of the spectrum, and so a complete curve can be 

 constructed which we call the illumination curve of the sj)ectrum of 

 the light under consideration. 



Now, when we are working in the laboratory with a steady light, 

 we may be at ease with this method, but when we come to working 

 with light such as the sun, in which there may be constant variation 

 owing to passing, and maybe usually imperceptible, mist, we are met 

 with a difficulty; and in order to avoid this, General Festing and 

 myself substituted another method, which I will now show you. We 

 made the comparison light part of the light we were measuring. 

 Light which enters the collimating lens partly passes through 

 the jDrisms and is partly reflected from the first surface of the 

 prism ; that we utilise, thus giving a second shadow. The reflected 

 rays from Pi fall on g, a silver-on-glass mirror. They are collected by 

 L5, and form a white image of the prism also at f. The method we 

 can adopt of altering the intensity of the comparison light is by 

 means of rotating sectors, which can be opened or closed at will, and 

 the two shadows thus made equally luminous. [Shown.] But 

 although this is an excellent plan for some purposes, we have found 

 it better to adopt a different method. You will recollect that the 

 brightest part of the spectrum is in the yellow, and that it falls off in 

 brightness on each side, so, instead of opening and closing the sectors, 

 they are set at fixed intervals, and the slit is moved in front of the 

 spectrum, just making the shadow cast by the reflected beam too dark 

 or too light, and oscillating between the two till equality is dis- 

 covered. The scale number is then noted, and the curve con- 

 structed as before. It must be remembered that, on each side of the 

 yellow, equality can be established. 



This method of securing a comparison light is very much better 

 for sun work than any other, as any variation in the light whose 

 spectrum is to be measured aflects the comparison light in the same 

 degree. Thus, suppose I interpose an artificial cloud before the slit 

 of the spectroscope, having adjusted the two shadows, it will be seen 



