OPTICS. 
and that each subsequent ray in the order 
above mentioned is more refrangible than 
the preceding. 
As a circular image would be depicted 
by the solar ray unrefracted by the prism, 
so each ray that suffers no dilatation by the 
prism would mark out a circular image, ij. 
Hence, it appears, that the spectrum is com- 
posed of innumerable circles of different 
colours. The mixture, therefore, is propor- 
tionable to the number of circles mixed to- 
gether (fig. 12); but all such circles are 
mixed together, whose centres lie between 
those of two contingent circles, consequently 
the mixture is proportionable to the inter- 
^ val of those centres, i. e. to the breadth of 
the spectrum. If therefore the breadth 
can be diminished, retaining the length of 
the rectilinear sides, the mixture will be 
<■ lessened proportionably, and this is done 
by the following process. 
At a considerable distance from the hole, 
%, place a double convex lens, AB (fig. 13), 
whose focal length is equal to half that dis- 
tance, and place the prism, x, behind the 
lens; at a distance behind the lens, equal to 
the distance of the lens from the hole, will 
be formed a spectrum, the length of whose 
rectilinear sides is the same as before, but 
its breadth much less ; for the undiminished 
breadth was equal to a line subtending, 
at the distance of the spectrum from the 
hole, an angle equal to the apparent dia- 
meter of the sun, together with a line 
equal to the diameter of the hole ; but the 
reduced breadth is equal to the diameter of 
the hole only; the image of the hole formed 
by the lens at the distance of double its 
focal length, is equal to the hole ; therefore, 
its several images in the different kinds of 
rays are equal to the same, i. e. the breadth 
of the reduced spectrum is equal to the 
diameter of the hole. 
A prism ABC, (fig. 14, Plate II.) 
placed in an horizontal position, would pro- 
ject the ray into an oblong form, as has 
been seen ; apply another horizontal prism, 
A D B, similar to the fonner, to receive 
the refracted light emerging from the first, 
and having its refracting angle turned the 
contrary way from that of the former. The 
light, after passing through both prisms, 
will assume a circular form, as if it had not 
been at all refracted, 
If the light emerging from the first prism 
be received by a second, whose axis is per- 
pendicular to that of the former, it will be 
refracted by this transverse prism into a po- 
sition inclined to the former, the red ex- 
tremity being least, and the violet most 
removed from its former position ; but it 
will not be at all altered in breadth. 
Close to the prism A (fig. 15), place a 
perforated board, a b, and let the refracted 
light (having passed through the small hole) 
be received on a second board, c d, parallel 
to the first, and perforated in like manner ; 
behind that hole in the second board place 
a prism, with its refracting angle downward, 
turn the first prism slowly about its axis, 
and the light will move up and down the 
second board ; let the colours be transmit- 
ted successively, and mark the places of 
the different coloured rays on the wall after 
their refraction by the second prism, the 
red will appear lowest, the violet highest, 
the rest in the intermediate places in order. 
Here then the light being very much sim- 
plified, and the incidences of all the rays 
on the second prism exactly the same; the 
red was least refracted, the violet most, &c. 
The permanency of these original colours 
appears from hence, that they suffer no 
manner of change by any number of re- 
fractions, as is evident from the last men- 
tioned experiment ; nor yet by reflection, 
for if any coloured body be placed in sim- 
plified homogeneeus light it will always ap- 
pear of the same colour of the light in 
which it is placed, whether that differ from 
the colour of the body or not ; e. g, if ul- 
tra marine and vermilion be placed in a red 
light both will appear red ; in a green light, 
green ; in a blue light, blue, &c. It is, 
however, to be allowed, that a body ap- 
pears brighter when in a light of its own 
colour than in another ; and from this we 
see that the colours of natural bodies arise 
from an aptitude in them to reflect some 
rays more copiously and strongly than 
others ; but lest this phenomenon should 
produce a doubt of the constancy of the 
primary colours, it is proper to assign the 
reason of it, which is this : that when placed 
in its own coloured light, the body reflects 
the rays of the predominant colour more 
strongly than any of those intermixed with 
it ; therefore the proportion of the rays of 
the predominant colour to those of the 
others, in the reflected light, will be greater 
than in the incident light ; but when the 
body is placed in a light of a different co- 
lour from its own, for’ a similar reason the 
contrary effect will follow, i. e. the propor- 
tion of the predominant colour to the 
others will be less in the reflected than in 
the incident light, and therefore as its splen- 
dor would be greater in the former case, 
