50 DR. JAMES BOTTOMLEY^S 



invested Tvitli clouds of various depths of grey, or sometimes 

 tinged by the sun with a variety of tints, from yellow to 

 red ; while the light of the sun itself is frequently yellow 

 or orange. All these variations of light are likely to have 

 some influence on our judgment of colour, especially when 

 the tints to be compared are light. Of the disturbing in- 

 fluence of colour in the incident light any one may convince 

 himself by comparing yellows on a morning when the sky 

 is enveloped in a yellow fog. In some experiments which 

 I made with bichromate of potash during such fogs I found 

 it much more difficult to decide at what depth equality of 

 colour was efiected ; the disk in the stronger solution could 

 be moved through a very considerable range without any 

 change of colour being perceived. A similar result 

 happened when I hung up yellow screens and tried to 

 make determinations of colour behind them; also when 

 looking at light-yellow external surfaces, difierences in the 

 lengths of the columns failed to give any difierences in tint, 

 although when looking at white external surfaces they did 

 so. But in quantitative determinations of matter by co- 

 lorimetry, the excellence of the results require sensible 

 variations in colour when we alter slightly the length of 

 the column ; hence, when the incident light is tinged with 

 the colour we wish to determine, the advantage of the 

 method is diminished. Such a consequence may also be 

 deduced from the formula which I obtained in my last 

 paper. For suppose white light to consist of yellow, blue, 

 and red (as far as the reasoning is concerned, we might 

 have considered it also composed of green, red, and violet, 

 as some physicists do) . Let I denote the incident white 

 light, and B, Y, R the intensities of blue, yellow, and red 

 necessary to produce white light, so that we may write 



I=B + Y-f-R; 



