S4 
at one end and yellow green at the other end, at the same time it 
will be very gradual, and so of the other tints, but there will be no 
browns: see Tab. 4. e. 
The outl'ne /. with the squares will show how any spot may be 
measured by length and breadth, as well as show the usual division 
of parts that constitute the best proportions for the most perfect 
colours. 
Some apology may here be necessary, on account of the difficulty 
of colouring the examples. It however may be urged, as in all other 
cases, that the most perfect artist, or the most perfect materials we 
borrow from Nature, resemble every thing else borrowed from that 
source, and we must refer to the Great Original alone for perfection. 
The use of the prism therefore is advisable to all that can have an op- 
portunity to jjrocure one ; and when I haveshown a means of forming a 
long wedge as most appropriate for seeing a large variety (see 
Tab. 7.) I shall proceed to explain it : and I may here observe, 
that it will be a fund of amusement to practise the producing of 
tints by these means, when perhaps something will be found out 
to improve and make it more instructive: An apparatus for thus 
measuring tints, or colours, I have called a Chromatometer. 
Having thus far spoken chiefly of prismatic colours, it may not 
be amiss to show the effect of mixing material colours which cor- 
respond exactly. 
Primary tints or colours I have considered as yellow, red and blue, 
because I do not comprehend that there has been proposed any 
way of compounding them from others, that I am not perfectly sure 
is founded on some deception ; and finding that they are capable of 
producing all others by mixture, in every method I have had oppor- 
tunity of examining with the greatest certainty. Venturing this con- 
sideration as conclusive, as well from the opinion of many that I con- 
sider competent Judges, as my own, I proceed to say something 
of the mixing them in two's or 
Binaries. Tab. 4. a. Now yellow No. 1 passing among red No. 2, 
