27 
tint, (which may be noted as a proof of the proper position of 
the jjrisra it is viewed by,) this will become more and more dilute to 
the end. The third j)art of the wedge begins where the second left 
off, with the same proportion and colour, and so of the other two ; 
as these diminish, the red diminishes, or becomes more and more 
dilute, till lost at the end of the fifth or last line of the wedge. The 
red, in the way I generally look at the prism, is at the bottom of these 
productors, as I may perhaps call them ; at the same time the blue at 
the top will diminish from full and perfect blue to the most dilute 
immediately opposite to the reds, and the yellow will do the same 
under the red, but is double the breadth, as represented in Tab. 4. 
This wedge therefore produces an infinite variety of these three pri- 
mitive tints, from the most full and perfect to the most dilute, which 
may be measured precisely at pleasure in a very certain manner ; so 
that every person may, in a common light, agree in pointing out a 
precise tint, even at distant parts of the world, if a similar wedge or 
chromatometer is used. Thus suppose the present to be a founda- 
tion, and for sake of accuracy was in general use, I would say, the 
red of Euphorbia Peplis for instance, (see English Botany, Tab. 
2002.) is, or should be, equal to one inch, that is equal to the red 
given by such a width of black on white paper ; and so of any tint 
or colour of any subject; and thus we may make conclusive com- 
parisons, and learn what was intended, though time had caused it 
to fade. 
CHROMATOMETER CONSIDERED SOMEWHAT ABSTRACTEDLY, 
Hitherto it has been chiefly shown how the prism may be used 
to produce colours in a book. It may now be necessary to show the 
use of the Chromatometer as an independent instrument (something 
like a barometer), as a useful and if we please an ornamental ap- 
pendage in a parlour or study, with a more exact method of apply- 
ing it. 
It might therefore be flattened on a board and hung like a pic- 
e2 
