that reason the glass R is still retained, to en- 
large the scope or area of the field. 
To find the magnifying power of this te- 
lescope, multiply the local distance of the 
great mirror by the distance of the small 
mirror from the image next the eye, and mul- 
tiply the local distance of the small mirror by 
the local distance of the eye-glass; then di- 
vide the product of the former multiplication 
*by that of the latter, and the quotient will 
express the magnifying power. The differ- 
ence between the Newtonian and Gregorian 
telescope is, that in the former the spectator 
looks in at the side through an aperture upon 
a plane mirror, by which the rajs reflected 
from the concave mirror are reflected to the 
eye-glass ; whereas in the latter the reader 
will see that he looks through the common 
eye-glass, which is in general more conve- 
nient. 
r l he immensely powerful telescopes of Dr. 
Ilerschel are oi a still different construc- 
tion. '1 his assiduous astronomer has made 
several specula, which are so perfect as 
to bear a magnifying power of more than six 
thousand times in diameter on a distant ob- 
ject. The object is reflected by a mirror as 
in the Gregorian telescope, and the rays are 
intercepted by a lens at a proper distance, so 
that the observer has his back to the object, 
and looks through the lens at the mirror. 
The magnifying power will in this case be the 
same as in the Newtonian telescope; but 
there not being a second reflector, the bright- 
ness of the object viewed in the Herschelian 
is greater than that in the Newtonian or 
G regorian telescope. In conclusion, sir Isaac 
Newton’s excellent maxim must not be 
omitted: “ The art,” says he, “ of con- 
structing good microscopes and telescopes 
may.be said to depend on the circumstance 
of making the last image as large and dis- 
tinct and luminous as possible.” 
There are some instruments of rather an 
amusing than a useful description, the effects 
ot which depend on a proper combination 
of plane or convex glasses. Our limits will 
not admit the notice of more than two of this 
kind, namely, the magic lanthorn, and the 
camera obscura. The former is a micro- 
scope upon the same principles as the solar 
microscope, and may be used with good ef- 
fect for magnifying sma l transparent objects ; 
but in general it is applied to the purpose of 
amusement, by casting the image of a small 
transparent painting on glass upon a white 
wall or screen, at a proper distance from the 
instrument. 
Let a candle or lamp C (fig. 8) be placed 
in the inside of a box, so that the light may 
pass through the plano-convex lens NN, and 
strongly illuminate the object OB ; which is 
a transparent painting on glass, inverted and 
moveable before NN, by means of a sliding 
piece in which the glass is set or fixed. This 
illumination is still more increased by the re- 
flection of light from a concave mirror SS, 
placed at the other end of the box, which 
causes the light to fall upon the lens NN, as 
represented in the figure. Lastly, a lens LL, 
fixed in a sliding tube, is brought to the re- 
quisite distance from the object OB, and a 
large erect image 1M is formed upon the op* 
posite wall. 
The camera obscura has the same relation 
to the telescope us the solar microscope has 
OPTICS. 
to the common double microscope, and is 
thus constructed: 
Let CD (fig. 12) represent a darkened 
chamber perforated at L, where a convex 
lens is fixed, ihe curvature of which is such, 
that the locus of parallel rays falls upon the 
opposite wall. Then if AB is an object at 
such a distance that the rays which proceed 
from any given point of its surface to the 
lens L may be esteemed parallel, an inverted 
picture will be formed on the opposite wall ; 
lor the pencil which proceeds from A will 
converge to a, and the pencil which proceeds 
from B will converge to b, and the interme- 
diate points of the object will be depicted 
between a and b. 
For the use of painters these instruments 
are now constructed in a very convenient 
mode. The lens is made to slide in a small 
wooden box, so as to be easily adjusted to a 
proper focus ; and the image falls upon a 
plane mirror, placed obliquely at the back 
part of the box, from which it is reflected on a 
piece of ground glass, or on a sheet of white 
paper extended over. The picture which is 
thus formed is very tender and beautiful, 
i he moving objects give it animation ; and 
the outline formed is so perfect that it mav 
be easily traced, even by a person who is 
little skilled in drawing or perspective. 
Of the doctrine of colours, or chromatics. 
— In some of the preceding sections we had 
occasion to use the word aberration, though 
we had not then an opportunity of explaining 
it ; since in the optics of the mind, as well as 
in those of which we are treating, when too 
many images are presented at once, a certain 
degree of confusion must necessarily ensue. 
As there is no “ royal road to science,” so 
philosophy gradually developes her secrets, 
and the possession of one fact prepares the 
mind for another. 
We have hitherto assumed as a principle, 
that a convex lens unites in one point, 
which we have called the focus, all the rays 
proceeding from any given point of an ob- 
ject. If this was exactly the case, the images 
formed by these glasses would be perfectly 
distinct and unconfused. The principle, 
however, holds strictly true only with respect 
to those rays which pass nearly through the 
centre ol the lens ; for those which pass near 
the extremities or edges of the glass, meet in 
foci still more distant, and from this multipli- 
cation of images great indistinctness results. 
To shew the reason of this it is necessary 
to have recourse to a figure. Let PP then 
(Plate III. fig. 10) be a convex lens; and 
Ee an object, the point E of which corre- 
sponds with the axb of the lens, and sends 
forth the rays EM, EN, EA, EM, and EN, 
all ot which reach the surface of the glass, 
but in different parts. Now it is manifest, 
upon the principles already explained, that 
the ray EA, which passes through the middle 
of the glass, suffers no refraction; the rays 
EM, EM, also, which pass through near to 
EA, will be converged to a focus at F, which 
we iiave been accustomed to consider as the 
locus of the lens. But the rays EN, EN, 
which are nearer to the edge of the glass, will 
be differently refracted ; and will meet about 
G, nearer to the lens, where they will form 
an ther image Gg. Hence it is evident that 
the first image I/is formed only by the union 
of those rays which pass very near the centre 
30 ? 
of the lens ; but, in truth, as the rays of light 
proceeding from every point of an ob ect are 
very numerous, there is a succession of 
images formed according to the parts of the 
lens where they penetrate, which necessarily 
produces great indistinctness and confusion ; 
and this is what is meant by the word aberra- 
tion. 
This confusion or dispersion of the rays is 
increased in proportion as the arcs PAP, 
PBP, are larger segments of their respective 
circles: hence in very thick and convex 
lenses the aberration is such as to be intoler- 
able. Even in the object-glasses of tele- 
scopes, though they are made thin, and are 
segments of large circles, and though from 
these reasons the dispersion of the rays may 
be insensible in itself, still the magnifying 
power multiplies it as often as the object it- 
self. lienee the greater the magnifying 
power, the smaller should be the aperture of 
the object-glass ; and when the dispersion of 
the rays is very great, the defect is in some 
degree remedied by covering the edge of the 
lens with an opaque ring ; but in this case, 
while distinctness is restored, the brightness 
of the image is necessarily diminished. Op- 
ticians have therefore endeavoured to form 
such combinations of lenses, both concave 
and convex, varying in their respective foci, 
as must unite all the rays in a single point, 
and thus present a distinct image. Calcula- 
tions have been formed for these combina- 
tions, but the hand of the artist has never 
been able to bring the speculations of theo- 
rists to entire perfection. 
The plan most generally adopted by prac- 
tical opticians is, to combine two shallow 
lenses together in such a manner that they act 
as a single lens. They use often plano-con- 
vex, for that figure admits of less aberration 
than any other ; but shallow lenses of a dou- 
ble-convex kind will answer. In this combi- 
nation the lenses are set near together, so that 
the second lens acts only in bringing the rays 
which pass through the first to a nearer focus. 
Thus in Plate III. fig. 9, AB and CD are 
two lenses of this description ; and the focus 
of AB would be at F, but, by the second 
lens, the rays are made to converge at a 
nearer focus/: thus they act together as a 
single lens of double their magnifying power, 
with this advantage ; that as the curvatures of 
both conjointly, are less than the curvature 
of a single lens of equal power, the aberration 
is greatly lessened. 
The aberration which we have been describ- 
ing results from the spherical form of the 
glasses ; but there is another kind of aberra- 
tion, which depends immediately upon the 
nature and properties of light itself. Each 
ray or beam of light, indeed, which gives us 
the sensation of white, is found to be com- 
pounded of seven other rays; and these com- 
ponent rays are each of them differently re- 
frangible. Hence objects viewed through 
very convex glasses are often found to have 
their edges tinged with various colours. This 
effect was long felt, but it remained for New- 
ton to explain the cause. 
In the short history contained in the first 
part of this article, the discoveries on colours 
were briefly related ; but it will perhaps be 
satisfactory to the reader to have the experi- 
ment described in the words of Newton him- 
self, which will at the same time afford an ex- 
