OPTICS. 
and would be less in the latter than if all 
the rays were equally reflected, the splen- 
dour of the predominant colour will be 
much greater in the former case than in the 
latter. 
As a solar ray was separated into several 
others of difierent colours, so, on the con- 
trary, from these homogeneous rays a ray 
of heterogeneous light may be compounded, 
perfectly corresponding both in appearance 
and properties with the solar rays. 
The colom-ed rays (fig. 16)^ diverging 
from the prism are received by a double 
convex lens, at tlie distance of twice its 
focal length from the hole ; at the same dis- 
tance behind the lens, where they are col- 
lected by its refraction, they are received 
on a second prism, whose refracting angle 
is equal to that of the former; the diver- 
gence of the homogeneous rays that would 
otherwise ensue, is counteracted by the 
second prism, and they are made to pro- 
ceed parallel to each other from the place 
of their intersection, and therefore are all 
compopnded and mixed together in the 
emergent ray A B, which is exactly of the 
same appearance with the solar rays, and, 
by experiments made on it similar to'those 
usually made in solar light, is found to pos- 
sess the same properties. 
Since then, 1. A solar ray may be re- 
solved into several differently coloured 
rays ; 2. Since their colours are immutable 
either by reflection or refraction, and there- 
fore probably not generated in those ope- 
rations ; and 3. Since from the mixture of 
those coloured rays solar light may be 
formed, it seems an indisputable, conclusion 
that the differently coloured rays do exist 
in solar light previously to any separation 
that takes place in experiments. 
White is compounded of all the pi imary 
colours mixed in their due proportions, for 
if a solar ray be separated by the prism 
into its component parts, and at a proper 
distance a lens be so placed as to collect 
the diverging coloured rays again into a 
focus, a papdr placed perpendicularly to 
the rays in this point will exhibit white- 
ness. 
The same conclusion may be drawn from 
the experiment of mixing together paints 
of the same colours as the parts of the spec- 
trum, and in the same proportion ; the mix- 
ture will be white, though not of a resplen- 
dent whiteness, because the colours mixed 
are less bright than the primary ones ; this 
may likewise be proved, by fixing pieces of 
cloth of all the seven difierent colours, on 
the rim of a wheel, and whirling it round 
with great velocity, it will appear to be 
white. Though seven different colours are 
distinguishable in the prismatic spectrum, 
yet, upon examining tlie matter with more 
accuracy, we shall see that there are, in 
fact, only three original colours, red, blue, 
and yellow ; for the orange being situated 
between the red and yellow, is only the 
mixture of these two; the green, in like 
manner, arises from the blue and yellow ; 
and the violet from the blue and red. 
As the colour of a body, therefore, pro- 
ceeds from a certain combination of the 
primary rays which it reflects ; the combi- 
nation of rays flowing from any point of an 
object will, when collected by a glass, ex- 
hibit the same compound colour in the cor- 
responding point of the image. Hence ap- 
pears the reason why tlie images formed by 
glasses have the colours of the objects which 
they represent. 
The instance of the separation of the 
primary colours of light which seems most 
remarkable, is that of the rainbow. It is 
formed, in general, by the reflection of the 
rays of the sun’s light from the drops of 
falling rain, though frequently it appears 
among the waves of the sea, whose heads, 
or tops, are blown by the wind into small 
drops, and it is sometimes seen on the 
ground, when the sun shines on a very thick 
dew. Cascades and fountains, whose wa- 
ters are in their fall divided into drops, ex- 
hibit rainbows to a spectator, if properly 
situated during the time of the sun’s shining; 
aud water blown violently from the mouth 
of an observer, whose back is turned to- 
wards the sun, will, with care, produce the 
same phenomenon. See Kainbow. 
This appearance is also seen by moon- 
light, though seldom vivid enough to render 
the colours distinguishable ; and the arti- 
ficial rainbow may be produced even by 
candle-light, on the water which is ejected 
by a small fountain, or jet d’eau. All these 
are of the same nature, and dependent on 
the same causes, viz. the various refrangibi- 
lity of the rays of light. 
The colours observable on soap-bubbles, 
and the halos which sometimes surround 
the moon, are also referable to the same 
origin, 
“ Of the Eye, and the Nature of Vision.” 
The eye is nearly of a spherical shape, and 
is composed of three different substances, 
called, 1, The aqueous, P (fig. 17). 2. 
The crystalline, R; aud 3. 'Ehe vitreous 
humours, V, enclosed by three principal 
