174 G. Tait, Esq., on produciug White or Neutral Light, 



light very weak. The expense of different lights will be no- 

 ticed presently. 



The artificial light is to be inclosed in a box or lantern 

 of a suitable size and construction, or otherwise, as circum- 

 stances may require, and in such manner that none of it may 

 escape by the openings for admitting and discharging air or 

 otherwise, except what is modified by passing through the 

 coloured medium. The modified light may be used either by 

 allowing it to radiate directly upon the objects to be enlight- 

 ened, or, in the case of transparent paintings, or the like, by 

 transmitting it through them. I have Used it satisfactorily in 

 both ways. 



As the Society are aware, the sensation of colours is caused 

 by objects reflecting or transmitting colours produced by the 

 rays of light to which they are exposed. There are three 

 primary colours, yellow, red, and blue ; of two, or all of which, 

 combined in various ratios, all other colours are composed. 

 Light, with very rare exceptions, is compound or heteroge- 

 neous, producing two or all of the primary colours. The pure 

 light of the sun produces them in such ratio, that when wholly 

 or proportionally reflected or transmitted, they neutralize each 

 other, and produce white or neutral grey (which is merely a 

 shade of white, having no predominant colour) ; and light 

 producing them in such ratio is often called shortly ivhite or 

 neutral light. In 100 parts of white, there appear to be about 

 18 parts of yellow, 32 of red, and 50 of blue, all of equal 

 intensity, or nearly as 3, 5, 8.* When two only of the pri- 



* Field, by mixing pure coloured liquids of equal intensity, and also by com- 

 bining hollow wedges of glass containing such liquids separately, and transmit- 

 ing daylight, found the ratio of the component parts of white to be 3 parts of 

 yellow, 5 of red, and 8 of blue. (See Chromatics, by George Field, 1845, pages 

 224, 231-2 ; Chromatography, by the same, 1835, p. 247.) Hay, by mixing pure 

 powdered pigments of equal intensity, foupd the ratio to be 3 parts of yellow, 

 6 of red, and 9 of blue. (See Nomenclature of Colours, &c., by D. R. Hay, 1845, 

 p. 8 ; The Principles of Beauty in Colouring systematized, by the same, 1845, 

 p; Vt.) The first series of numbers contained in the text, are the average of 

 those three sets of observations; whicli coincide, with the exception of a slight 

 difference in the ratio of the yellow and the red, arising perhaps, from the 

 difficulty of distinguishing whether either of those colours is entirely free from 

 a tinge of the other. 



