unknown to the other. He shews, more- ■ 
over, that a defect of light is remedied by the ! 
dilatation of the pupil, which contracts mvo- 
luntarily when exposed to a strong light, and 
opens when the light is faint and languid. 
One Fletcher, of Breslau, in 1571, endea- 
voured to account for the phenomena of the 
rainbow, by a double reflection and one re- 
fraction ; but Antonio de Bominis, whose 
treatise was published in loll, was the first 
who came near to the true theory. lie de- 
scribes the progress of the ray of light through 
each drop of the falling rain ; he shews that 
it enters the upper part of the drop, where 
it suffers one refraction; that it is reflected 
once, and then refracted again, so as to come 
directly to the eye of the spectator: why this 
refraction should produce the different co- 
lours, was reserved for sir Isaac Newton to 
explain. 
The latter part of the sixteenth century 
was illustrious for the invention of telescopes. 
It is generally allowed to have been casual. 
That effect of refraction, which causes the 
rays of light, in passing through a dense me- 
dium thicker in the middle, to converge to a 
point, and also that which takes place when 
they pass through one thicker at the extremi- 
ties, had been long observed ; and the assist- 
ance which convex and concave glasses af- 
forded to the sight, had brought them into 
common use ; 'The inventor of the telescope 
is not certainly known. The most probable 
account is, that one Zacharias Jansen, a spec- 
tacle-maker ef Middleburgh, trying the ef- 
fect of a concave and convex glass united, 
found that, placed at a certain distance from 
each other, they had the property of bring- 
ing distant objects apparently nearer to the 
eye. An account which is very commonly 
received, is, that some of his children playing 
in his shop with spectacle-glasses, perceived 
that when they held two of these glasses be- 
tween their fingers, at a certain distance from 
each other, the dial of the clock appeared 
greatly magnified, but in an inverted posi- 
tion. From this their father adopted the idea 
of adjusting two of these glasses on a board, 
so as to move them at pleasure. Telescopes 
were greatly improved by Galileo, who con- 
structed one which magnified 33 times, and 
with this he made all his wonderful astrono- 
mical discoveries. 
The rationale of telescopes was,. however, 
not explained till Kepler, who described the 
nature and the degree of refraction, when 
light passed through denser or rarer mediums, 
the surfaces of which are convex or concave ; 
namely, that it corresponds to the diameter 
of the circle of which the convexity or conca- 
vity are portions of arches. He suggested 
some improvements in the construction of te- 
lescopes, which, however, were left to others 
to put in practice. 
To the Jansens we are also indebted for 
the discovery of the microscope; an instru- 
ment depending upon exactly the same prin- 
ciples as the former. In fact, it is not impro- 
bable, that the double lens was first applied 
to the observation of near but minute objects, 
and afterwards, on the same principles, to 
objects which appeared minute on account of 
their distance. 
Much attention was given by Kepler to 
the investigation of the law of refraction ; but 
Ire was able to advance no nearer the truth 
OPTICS. 
■ than the observation, that when the incident 
! ray does not make an angle of more than 30 
| degrees with the perpendicular, the refracted 
ray proceeds in an angle which is about two- 
thirds of it. Many disputes arose about the 
time of Kepler (lbOO) upon this subject, but 
it appears that little was effected by them in 
the cause of truth. 
Kepler was more successful in pursuing 
the discoveries of Mau roly cus and B. Porta" 
lie demonstrated that images of external ob- 
jects were formed upon the optic nerve bv 
the foci of rays coming from every part of 
the object: he also observed, that these 
images are inverted; but this circumstance, 
he says, is rectified by the mind, which, when 
an impression is made on the lower part of 
the retina, considers it as made by rays pro- 
ceeding from the higher parts of the object. 
Habit is supposed to reconcile us to this de- 
ception, and to teach us to direct our hands 
to those parts of objects from which the rays 
proceed. Tycho Brahe, observing the appa- 
rent diminution of the moon’s disc in solar 
eclipses, imagined that there was a real di- 
minution of the disc by the force of the sun’s 
rays; but Kepler said, that the disc of the 
moon does not appear less in consequence of 
being unenlightened, but rather that it appears 
at other times larger than it really is, in con- 
sequence of its being enlightened. For pen- 
cils of rays from such distant objects gene- 
rally come to their foci before they reach 
the retina, and consequently diverge and 
spread when they reach it. For this reason, 
he adds,, different persons may imagine the 
disc to be of different magnitudes, accord- 
ing to the relative goodness of their sight. 
In the sixteenth century also many im- 
provements were made in perspective ; the 
ingenious device, in particular, of the refor- 
mation of distorted images by concave or 
convex speculums was invented, but it is 
uncertain by whom. 
The true law of refraction was discovered 
by Snellius, the mathematical professor at 
Leyden; but not living to complete it, the 
discovery was published and explained by 
professor Hortensius. Some discoveries of 
lesser importance were made at this time, 
among others by Descartes, who very clearly 
explained the nature and cause of the figure 
of the rainbow, though he was able to give 
ro account of the colours; he however con- 
sidered the small portion of water, at which 
the ray issues, as having the effect of a prism, 
which was known to have the property of 
exhibiting the light, transmitted through it, 
coloured. 
In 1625, the curious discovery of Schemer 
was published at Rome, which ascertains the 
fact, that vision depends upon the images of 
external objects upon the retina. For taking 
the eye of an animal, and cutting away the 
coats of the back part, and presenting differ- 
ent objects before it, he displayed their images 
distinctly painted on the naked retina or 
optic nerve. The same philosopher demon- 
strated by experiment, that the pupil of the 
eye is enlarged in order to view remote ob- 
jects, and contracted when we view those 
which are near. Fie shewed, that the rays 
proceeding from any object, and passing 
through a small hole in a pasteboard, cross 
one another before they enter the eye ; for if 
the edge of a knife is held on the side next 
the eye, and is moved along till it in part 
2 93 
covers the hole, it will first conceal from the 
eye that part of the object which is situated 
on the opposite side of the hole. 
Towards the middle of the seventeenth 
century the velocity of light was discovered 
by some members of the Royal Academy of 
Sciences at Paris, particularly Cassini and 
Roemer, by observing the eclipses of Jupiter’s 
satellites. About the same time Mr. Boyle 
made his experiments on colours. He proved 
that snow did not affect the eye by a native, 
but reflected light, a circumstance which, 
however, at this day, we should scarcely be- 
lieve was ever necessary to be proved by 
experiment. By admitting also a ray of light 
into a dark room, and letting it fall on a 
sheet of paper, he demonstrated, that white 
reflected much more light than any other 
colour; and to prove that white bodies reflect 
the rays outwards,, he adds, that common, 
burning-glasses wiil not, for a long while, bum 
or discolour white paper; on the contrary, a 
concave mirror of black marble did not re- 
flect the rays of the sun with near so much 
power as a common concave mirror. The 
same effect was verified by a tile, one half of 
the surface of which was white, and the other 
black. 
Some experiments were made about this 
time on the difference of the refractive powers 
of bodies ; and the first advance to the great 
discoveries by means of the prism w as made 
by Grimaldi, w'ho observed, that a beam of 
the sun’s light, transmitted through a prism, 
instead of appearing round on the opposite 
wall, exhibited an oblong image of the sun.. 
Towards the close of this century the reflect- 
ing telescope was invented by our country- 
man James Gregory. It was, however, only 
an idea conceived by him upon theory, anci 
the first reflecting telescope was made by 
Newton. 
The reader will soon perceive how verv 
imperfect all the preceding discoveries were 
in comparison with those of sir Isaac: New- 
ton. Before this time, little or nothing was 
known concerning colours; even lire remark 
of Grimaldi respecting the oblong figure of 
the sun, made by transmitting the rays 
through a prism, was unknown to our great 
philosopher, having been published only the 
year before. This fact, however, which he 
had observed himself, w r as, it appears, the 
first circumstance which directed the atten- 
tion of Newton to the investigation of the- 
theory of colours. Upon measuring the 
coloured image, which was made by the 
light admitted into a dark chamber through 
a prism, he found that its length w r as five 
times greater than its breadth. So unac- 
countable a circumstance induced him to try 
the effect of two prisms, and he found that 
the light, which by the first prism was diffused 
into an oblong, was by the second reduced to 
a circular form, as regularly as if it had passed 
through neither of them. After many con- 
jectures and experiments relative to the cause 
of these phenomena, lie at length applied to 
them what he calls the experimentwn crucis. 
He took two boards, and placed one of them 
close to the window, so that the light might 
be admitted through a small hole made in it, 
and after passing through a prism might fall 
on the other board, which was placed at 
about tw elve feet distance, and in which there 
was also a small aperture, in order that some. 
