2 INTRODUCTION. 
be found out. I therefore determined, if in my power, to find it 
oat ; and was much sooner favoured with the discovery than I ex- 
pected, and that too to a nicety that surprised me much, especially 
as it was so easy and likely to be brought to perfection, not only to 
correct former errors, but to serve as a firm basis for future inquiries. 
Indeed, it may seem a wonder, in the present rapidly improving 
age, that any thing remained to be done towards the elucidation of 
colours, either evanescent or natural. I presume, however, that this 
work in its present form is likely to fill up some important desi- 
derata ; and if it is not established as a book of reference, which is 
much wanted, I hope it will help at least to call one forth, as such 
a book may be of infinite service in many departments of science, 
for the subject promises already to branch into many divisions for 
different purposes. One truth established helps another ; and it 
may give a hint even to the astronomer Dr. Herschel, who has 
paid so much attention to the subject of prismatic colours, that 
strong opposites, such as light and dark, may not be proper in the 
use of telescopes, as when they are next each other, in particular, 
they may produce the most brilliant prismatic tints by the help of 
the angle of a lens*. Thus a middle tint I should presume best 
for lining telescopes. I find a middle tint in my schemes best to 
neutralize colours (black I believe is in general use for telescopes 
kc. but 1 do not know what Dr. Herschel uses). 
Again, calculation by means of a prism has been used to assist 
in examining some of the constellations ; if the sun's width at a 
known distance is such as to show a white space when viewed in a 
prism, it must be broader or nearer than those constellations that do 
not show a white space: — thus the smaller objects of our hemisphere 
show only the four colours of Dr. Wollaston's narrow ray of light, 
or C. A. Prieur's white thread or silk, which will be understood in 
the sequel. It is somewhat curious, that the middle tints brought in 
contact by smallness or narrowness exclude light or white, and be- 
* Dr. Herschel probably broke his coloured glasses by some such contrast. 
