ON COLOUR PHOTOMETRY. 551 



mode by noting if the colours of the two shadows were of exactly the same hue, 

 more especially in the transition between orange and green, and in that from the 

 blue-green to blue. The slightest departure from true adjustment invariably showed 

 itself in these two parts of the spectrum. We should not hesitate to adjust the 

 instrument by this means alone, though in all the measures taken the comparison of 

 the two spectra on white card was invariably made. 



XXXI. Exclusion of Extraneous Light. 



There is another point of special importance to be attended to, viz., the exclusion of 

 all extraneous light from the receiving screen. If two shadows are to be compared 

 together, when the whole of the screen is white or of the same colour, the admission 

 of extraneous light is not detrimental ; but, if one shadow falls on a white ground and 

 the other on what in white light is a coloured ground, it is absolutely necessary to 

 keep the screen free from all light except that forming the shadows. 



It was curious to note the change in colour produced on the coloured half of the 

 screen when illuminated partially by a portion of the spectrum weak in luminosity, and 

 partially by weak white light. It was absolutely impossible to match the colours, 

 when even a very small percentage of white light fell on the screen. The whole 

 apparatus was placed in a darkened room, the electric light being in a lantern. 

 Extraneous light was excluded by placing the screen at the end of a box 18 inches 

 wide, 12 inches deep, and 2 feet long, the interior being blackened. A white card 

 placed at the end of the box was then invisible when the electric light was burning 

 and the slit in the card was placed beyond the limit of the spectrum. 



XXXII. Width of Slit Employed. 



The great point in measuring accurately was to adjust the luminosity so that it was 

 of such brightness that the eye could readily distinguish any small difference in the 

 brightness of the illuminated shadows. This was effected by altering the width of 

 the slit in the collimator from time to time. When the brightest part of the spectrum 

 was under measurement, the width was about t^th of an inch, and, when the least 

 luminous parts, it was opened to about -j^-th of an inch. The slit in the card remained 

 invariable, being about -^jth of an inch in width. The screen was placed 3 feet from 

 the slit card. 



XXXIII. Experiments with Emerald Green, Vermilion, and Ultramarine. 



The first experiments were conducted to ascertain the composition of the grey light 

 given by a set of discs of emerald green, vermilion, and French ultramarine. Discs of 

 these colours, 6 inches in diameter, were prepared ; and a larger pair of black and 

 white discs arranged on the same axis. The sectors of the three colours and of the 



