70 GORHAM, ON THE 
The facilities of mixture afforded by this process, as con- 
trasted with that of ordinary mixture, by the amalgamation 
of the pigments with water or oil, are as follows: The 
colours used are chosen once for all from amongst the purest 
of the pigments; they are laid on circular discs of card- 
board in intense washes, and thus they may be used again 
and again as occasion requires. They are few in number, 
because few only are required ; they are mixed im all propor- 
tions evenly and smoothly with perfect ease by mere rota- 
tion; they are cancelled at pleasure, even during rotation, 
by scales constructed for the purpose, and they are liberated 
by the same process, just as the sounds from the pipes of an 
organ are stopped or unstopped by touching the keys. The 
relative quantities of colour entering into given compounds, 
moreover, can be expressed numerically by reference to a 
scale of degrees affixed to the wheel, thus enabling us to 
name a colour in reference to its constituents with some de- 
gree of philosophical accuracy. It is important to notice, 
however, that while the results of mixture by the ordinary 
process and by rotation bear on the whole a striking resem- 
blance, a remarkable exception obtains with respect to the for- 
mation of green. This hue is produced in the ordinary way, 
as is well known, by the union of yellow and blue in almost 
any proportions; not so by rotation, for by a curious 
anomaly, there is not a yellow and blue in existence, com- 
bined in any proportions, that will form even a tolerable 
green. 
With a graduated scale enabling us to express areas of 
coloured space numerically, with intense washes of pure 
colours, and with a given velocity, the important problem of 
a nomenclature of colours would appear to be solved; but 
the known impurity of every pigment, and our inability to 
produce a green hue by rotation, conspire to form an insu- 
perable obstacle, and the construction of a nomenclature by 
this process, although possible, must be relinquished as 
useless. 
The combinations by rotation serve to illustrate many of 
the most interesting phenomena of colour; they furnish a 
clue, for instance, to the theoretical composition of the 
pigments, elucidate the principles of contrast, evoke the 
complementaries, and enable us to blend colours in softer 
gradations than we can by the pencil. These results are 
obtained indeed with so much ease and certainty, that this 
mode of studying colour might, it is presumed, be adapted 
with success to educational purposes. 
I propose to divide this part of the subject under the fol- 
