308 
ample of the style and manner of this first of 
philosophers. 
“ In a very dark chamber, at a round hole 
F (Plate III. fig. 14), about one-third of an 
inch broad (says he), made in the shutter of a 
window, I placed a glass prism ABC, whereby 
the beam of t he sun’s light, SF, which came 
in at that hole, might be refracted upwards, 
toward the opposite wall of the chamber, and 
there form a coloured image of the sun, re- 
presented at PT. The axis of the prism 
(that is, the line passing through the middle 
of the prism, from one end of it to the other 
end, parallel to the edge of the refracting an- 
gle) was in this and the following experiments 
perpendicular to the incident rays. About 
this axis 1 turned the prism slowly ; and saw 
the refracted light on the wall, or coloured 
image of the sun, first to descend, and then 
to ascend. Between the descent and ascent, 
when the image seemed stationary, I stopped 
the prism, and fixed it in that posture. 
“ Then I let the refracted light fall perpen- 
dicularly upon a sheet of white paper, MN, 
placed at the opposite wall of the chamber; 
and observed the figure and dimensions of the 
•solar image PT, formed on the paper by that 
light. This image was oblong, and not oval, 
but terminated by two rectilinear and paral- 
lel sides, and two semicircmar ends. On its 
sides it was bounded pretty distinctly ; but on 
ifs ends very confusedly .and indistinctly, the 
light there decaying and vanishing by de- 
grees. At the distance of 181 feet* from the 
prism, the breadth of the image was about 
2$ inches, but its length was about lOf inches, 
and the length of its rectilinear sides about 8 
inches ; and ACB, the retracting angle of the 
prism, whereby so great a length was made, 
was 64k With a less angle the length of 
tire image was less, the breadth remaining the 
same. It is farther to be observed, that the 
rays went on in straight lines from the prism 
to the image ; and Therefore at their going 
out of the prism had all that inclination to 
one another from which the length of the 
image proceeded. This image P I was co- 
loured, and the more eminent colours lay in 
this order from the bottom at T to the top at 
P; red, orange, yellow, green, blue, indigo, 
violet, together with all their intermediate 
degrees, in a continual succession, perpetu- 
ally varying.” 
The philosopher continued his experi- 
ments, and by making the rays thus decom- 
pounded pass, aswas formerly related, through 
a second 'prism, he found that they did not 
admit of farther decomposition ; and ihat ob- 
jects placed in the rays producing one colour 
always appeared to be of that colour. He 
then examined the ratio between the sines of 
incidence and refraction of these decom- 
pounded rays ; and found that each of the 
seven primary colour-making rays, as they 
may be. called, had certain limits within 
which they were confined. Thus, let the 
sine of incidence in gdass be divided into 
fifty equal parts, the sine of refraction into 
air* of the least and most refrangible rays 
will con ta n respectively 77 and 78 such 
parts, fhe sines of refraction of all the de- 
grees of red will have tne intermediate de- 
grees of magnitude, from 77 to 77f ; orange 
from 77f to 771. ; yellow from 77 | to 77f; 
green from 7/y to 77^ » blue from //a to 
OPTICS. 
77 J ; indigo from 77J to 77^. ; and violet 
from 77^. to 78. 
According to the properties of bodies in 
reflecting or absorbing these rays, the co- 
lours which we see in them are formed. If 
every ray falling upon an object was reflected 
to our eyes it would appear white; if every 
ray was absorbed it would appear black ; be- 
tween these two appearances innumerable 
species of colours may be formed by reflection 
or transmission of the various combinations 
of the colour-making rays. If the rays also 
of light were not thus compounded, every 
object would appear of the same colour, and 
an irksome uniformity would prevail over 
the face of nature. 
To leave, however, for the present, tire 
further prosecution of this subject, and to 
return to that of the errors arising in optical 
glasses from the dispersion of the rays of light, 
it must be evident that, in proportion as any 
part of a glass bears a resemblance to the 
form of a prism, the component rays must be 
necessarily separated. The edges of every 
convex lens approach to this form ; and it is 
on this account that the extremities of objects 
viewed through them are found to be tinged 
with coloured rays. In reality, as all the dif- 
ferent colour-making rays are differently re- 
frangible, in such a glass these different rays 
will have different foci, and will form their 
respective, images at different distances from 
the glass. Thus imagine PP (Plate III. 
fig. 11) to be a double-convex lens, and 00 
an object situated at some distance from it. If 
the object 00 was red, the rays proceeding 
from it would form a red image at Rr; if it 
was violet, an image of that colour would be 
formed at Vv nearer the glass ; and if the ob- 
ject was white, or any other combination of 
the colour-making rays, these rays would 
have their respective foci at different dis- 
tances from the glass, and form a succession 
of images, in the order of the prismatic co- 
lours, between the space Rr and Vv. 
This dispersion depends on the focal length 
of the glass, the space which the coloured 
images occupy being about the 28th part. 
Thus, if the glass is of 28 feet focus, the 
space between Rr and Vv will be about one 
foot, and so in proportion. Now when view- 
ed through one eye-glass or more, this suc- 
cession of images will seem to form but one 
image, but that very indistinct, and tinged 
with various colours ; and as the red image 
Rr in the figure is largest, or seen under the 
greatest angle, the extreme parts of this con- 
fused image will be red, and a succession of 
the prismatic colours will be formed with this 
red fringe, as is frequently found in telescopes 
upon the old construction. 
This defect in telescopes was long regarded 
as without a remedy ; but who shall set 
bounds to the inventive powers of the human 
mind ? It was in the different refractive 
powers of various media that a remedy 
was sought for this property in glasses, so ad- 
verse to tne hopes and wishes ot philosophers. 
Sir Isaac Newton had hinted the practicabi- 
lity of this plan ; but he was too deeply en- 
gaged in the vast discoveries which the use 
of the reflector opened to his view, to pur- 
sue practically the idea. As water is known 
to have very different refractive pow. rs from 
glass, the great Euler, proceeding upon tint 
hint of Newton, projected an object-glass of 
two lenses, with water between them. The 
memoir of Euler excited powerfully the at- 
tention of Mr. Dollond, a practical optician 
in London ; and after trying the refractive 
power of water combined with glass in the 
form of a prism, lie conceived- tnat the re- 
fractive powers of different glasses might 
serve to correct each other. He app ied 
himself therefore to examine the qualities of 
every kind of g'ass he could procure, aacl 
found that the two which differed most es*- 
sentially in their refractive powers were t lie- 
common crown or window glass, and the 
white flint glass. He then formed two prisms, 
one of the white flint of an angle of about 25 
degrees, and another of flint of 29. They, 
refracted very nearly alike, but their power 
of making the colours diverge was very dif- 
ferent. lie next ground several others of 
crown glass, till he procured one which was 
equal as to the divergency of light with that 
of the flint glass. He placed them together, 
therefore, but in opposite directions, so as to 
counteract each other ; and he found tiiat the 
light which passed through them was per- 
fectly white. 'Fliis discovery, it was obvious, 
was immediately applicable to the object- 
glasses of telescopes. To make the glasses 
act as the two prisms, to refract the light in 
contrary directions, it was plain that the one 
must he concave and the other convex -. and 
as the rays are to converge to a real locus, the 
excess of refraction ma t be in the convex 
lens. As the convex lens is to refract most 
also, it appeared from his experiments that it 
must be of crown glass. He therefore em- 
ployed two convex lenses of crown glass, 
with a concave lens of flint glass ; and these 
are the telescopes most in use at present, and 
well known by the name of achromatic te- 
lescopes. Some opticians however, we. be- 
lieve, now construct them with two lenses, one 
convex and the other concave. 
In fig. 13, a and c shew the two convex 
lenses, and bb the concave one, of this tele- 
scope. They are all ground to spheres of dif- 
ferent radii, according to the refractive pow- 
ers of the different kinds of glass, and the in- 
tended focal distance of the object-glass of 
the telescope. According to Boscovich, the 
focal distance of the parallel rays for the con- 
cave lens is one-half, and for the convex 
glass one-third, of the combined focus. When 
put together thev refract the rays in the fol- 
lowing manner: Tet ab, ab (fig. 18), be two 
red rays of the sun’s light falling parallel on 
the first convex lens c. Supposing there was 
no other lens present but that one, they would 
then be converged into the lines be, be, and 
at last meet in the focus q. Let the lines 
gh, gh, represent two violet rays falling on 
the surface of the lens. These are also re- 
fracted, and will meet in a focus ; but as they 
have a greater degree of refrangibility than 
the red rays, they must of consequence con- 
verge more by the same power of refraction 
in the glass, and meet sooner in a focus, sup- 
pose at r. Let now the concave lens of flint 
glass dd be placed in such a manner as to in- 
tercept all the rays before they come to their 
focus. If this lens was made of the same 
materials, and ground to to the same radius 
with the convex one, it would have the same 
power to cause the rays to diverge that the 
former had to make them converge. In 
this case, the red rays would become paral- 
