400 Proceedings of the Royal Institution, 



plemeiitary colours is generally known. Mr. Brockedon's mode of 

 readily distinguishing them, we believe, is new. In the usual way, 

 an equilateral triangle is made, each line is tinged or painted one of 

 the three primitive colours, red, blue, yellow. Where these lines 

 intersect, the red and yellow, overlying each other, produce orange ; 

 the red and blue, purple ; and the blue and yellow, green. Draw 

 this diagram as you will, the compound of any two primitives is 

 always opposed to the third primitive. On the outside of this triangle 

 a broad circle is drawn, and on the lines in this circle, which radiates 

 from the centre of the six simple and compounded colours, these 

 colours are painted, and each then blended into the other, so that a 

 deviation of red towards yellow is more and more a yellowish red, 

 until it become orange in the middle, between the red and yellow ; 

 the yellow, still increasing, becomes more and more yellow, until any 

 red ceases, and on the yellow radius it is pure yellow. This then 

 passes into green deeper and deeper, by accessions of blue ; the blue 

 into purple, by accessions of red, until the circle is completed in pure 

 red. Upon this broad ring of blended colour place a card, revolving 

 upon a common centre with the coloured ring, and having two holes 

 cut diametrically opposite to each other, and opening upon the co- 

 loured ring below. Wherever this is placed, those colours which 

 are seen through the holes would, if combined in coloured light, (not 

 in the foul pigments of painters,) produce white, or colourless light. 

 Thus, if the red and green are seen, on turning the upper card disk 

 as the red became purpler, the green became in the opposite hole 

 yellower, until perfect purple opposes perfect yellow, and so on. 

 After gazing for a short time upon any colour presented through one 

 hole, if a piece of paper be suddenly placed over it, the form of the 

 hole will still be presented to the sensorium ; but of the colour which 

 may be seen in the opposite hole in the diagram. There are many 

 applications of a knowledge of this fact ; some, which are familiar, 

 though before unnoticed, were exhibited by Mr. Brockedon. Co- 

 loured posting-bills printed in black ink, always excite, in a good 

 light, the idea of the ink being not black, but the converse colour of 

 the paper upon which they are printed; or, if the black letters be cut 

 out, and a piece of white paper pasted behind, so as to appear like 

 white letters, these present also the same contrast of colour : thus 

 on a yellow paper, the black letters appear a deep purple, and the 



