496 JOURNAL OF THE PLYMOUTH INSTITUTION. 
The tendency to suppose the colours of different lights an indi- 
cation of some distinctive quality residing in the light also re- 
ferred to as erroneous. All the colours of homogeneous lights 
may be more or less compound sensations, and that mixtures of 
different kinds of light may have the same colours. Colours of 
certain kinds of homogeneous light may be imitated by mixtures 
of other kinds, and the very same kinds of light may have dif- 
ferent colours, not only in different eyes, but also under different 
circumstances, or by lapse of time in the same eye. Field's plan 
of using a combination of hollow glass wedges, filled with coloured 
solutions, as a means of measuring the intensity of a colour ac- 
cording to the thickness of the solutions, which give a colour to 
match it, was shown to be fallacious. To thicken a solution or 
transparent pigment alters the hue as well as darkens its colour. 
Colouring matters act by destroying some kinds of light faster 
than others, so that light that escapes from thick washes of a 
transparent pigment cannot be the same in quantity or quality. 
White, as differing from other colours, described as containing all 
the simple colour sensations in equal strength. Maxwell's, J. J. 
Muller's, and Helmoltz's experiments were described as leading to 
the same results in the consideration of the colour sensations. 
Their investigations prove that both white and all possible colours 
can be produced by mixtures of the best prismatic red, green, and 
blue, and by mixtures of no other colours. The different means 
of producing the spectrum for the study of colours were described. 
An experiment presenting to the eye the prismatic colours in 
their greatest purity merging into darkness, also the colours of 
all possible parcels of continuous prismatic rays, was shown, being 
produced by viewing figures represented on a white ground. 
The means of determining the colours which lie between two 
given colours were as follows : One by Newton's disc revolving ; 
and by placing spots of the colours to be mixed laid a little dis- 
tance apart upon a neutral ground equally illuminated, a clean 
slip of thin polished glass is held vertically between them, the 
reflection of the one may be made to fall upon that part of the 
glass through which the other is seen. The more obliquely the 
light impinges on the glass, the more light is reflected, and the 
less transmitted ; so that by lowering the eye from a position 
high and near to the plane of the glass to a position opposite to 
its lower part, every colour intermediate between the colours of 
