32 P. C. on the Colours that enter into the [JulV 



mentary colours form white light with it, and they disap- 

 pear, leaving the colour or colours which happen to be in 

 excess, diluted with the white light thus formed. A red- 

 blue, a violet-yellow, or a green crimson is, therefore, 

 impossible. It is upon this principle that when blue and 

 yellow are superposed, green makes its appearance, there 

 being a surface of this colour combined with violet to form 

 the blue, and another surface combined with red to form 

 the yellow, a mixture of the two colours must necessarily 

 leave a surface of green in excess. 



These views lead to considerations of importance to 

 several of the arts ; but having already extended this paper 

 beyond the limits I had prescribed to myself, I shall reserve 

 this part of the subject for another opportunity. 



The theoretical views derived from the appearance of the 

 spectrum in different stages of its developement, may be 

 illustrated by taking three cards, the size of the face of the 

 prism, of the three colours, violet, green and red ; and, 

 after placing them with their edges parallel, which will 

 represent the superposition of all the colours to form the 

 white light incident upon its surface, gradually raise the 

 violet and the green cards, the former twice as much as the 

 latter, which will produce a separation of the colours nearly 

 in the proportion they are separated, by gradually increased 

 distance after refraction ; the parts of the cards which 

 remain superposed will exhibit the different combinations 

 as they appear on the screen at different distances from the 

 prism. Where the three cards are together, the part of the 

 spectrum they represent is white ; the green and red, from 

 which the violet is withdrawn, form yellow ; the green and 

 violet, from which the red is withdrawn, blue ; and the full 

 developement of the spectrum is shewn by the complete 

 separation of the three cards, (see figures 1 & 2). 



I am not acquainted with any writer who has noticed the 

 appearance of the spectrum previous to its full develope- 

 ment except Ritter, who did not appear to comprehend its 

 character, but considered it, when received on a screen a 

 few inches from the prism as forming two spectra, which 

 at greater distances became united.* 



If you consider the subject deserving a place in the 

 Records of Science, I intend, in a future communication, to 



• I quote from memory, having no book with me to refer to. 



