Observattons on Colours. 197 
These carry forward the angular fringes formed by reflection, 
which being refracted towards the perpendicular fall on the spec- 
trum H. The red and yellow rays, passing through the thin 
angle A, must be more heated, when falling on the spectrum H, 
than the blue rays passing through the angles B. C. 
Sir, I beg leave to remain your obedient servant, 
JosrPH Reapz, M.D. 
*,* In the communication, volume xlii. p. 418, Dr. READE’S 
name was printed Reaper, which our friends are requested to 
correct with a pen.— Ebr. 
eee eee 
XXXIX. Observations on Colours, as applicable to the Purposes 
of the Artist. By Mr. Tuomas HaRGREAVEs. 
Liverpool, Dec. 12, 1813. 
Sirs, 3) j TAKE the liberty of addressing to you some observa- 
tion on colours, the result of great attention to the subject, and 
of long experience as an artist, which may perhaps be useful for 
the purpose suggested by Mr. Forster, in his communications 
inserted in your last and the preceding numbers. 
I agree with him in the theory of three primary colours only, 
and with his account of the effects of the binary and ternary 
compounds ; but his idea of the precise tint of one of those pri- 
mary colours is certainly incorrect, from his having adopted the 
prismatic scale, which is defective, as I trust will be evident in 
the course of the following remarks. 
Before I proceed, I think the annexed figure (fig. 2. Plate III.) 
will assist me in my explanations. In this figure the three sim- 
ple primary colours, red, yellow, and blue, are placed at the three 
angles of a triangle, and the three intermediate colours, orange, 
green, and purple, in their proper situations between the respec- 
tive colours which compose them, and at the angles of another 
triangle. The space between a primary and intermediate com- 
pound is likewise marked by the two letters on each side: thus, 
between yellow and orange is put yo, to signify yellow orange, 
or an orange colour approaching to yellow. It is evident that 
the simple colours are like mathematical points, neither extend- 
ing one way nor the other; but an intermediate colour has an 
infinite variety of tint, from the verge of one of its components to 
that of the other. White and black, with all the intermediate 
degrees of gray, are equal combinations of the three simple co- 
lours. Pure white, by uniting all the colours, can only be pro- 
duced with the rays of light, as shown by the prism ; but all the 
different degrees of broken white or gray, down to black, may be 
produced by the coloured substances used in painting. An equal 
N3 portion 
