OPTICS. 
ceriumg tin's wonderful fluid. It has been 
'sometimes considered as a distinct substance, 
sometimes as a quality, ’sometimes as a cause, 
frequently as an effect ; by some regarded as 
a compound, and by others as a simple sub- 
stance. Descartes and other philosophers 
of high repute, have imagined that the sen- 
sation which v.e receive from light is to be 
attributed entirely to the vibrations of a sub- 
tile medium or fluid, which is diffused through- 
out the universe, and which is put into action 
by the impulse of the sun. In this view they 
consider light as analogous to sound, which i 
is known to depend entirely on the pulsations 
of the air upon the auditory nerves; and in 
support of this opinion, it has been even late- 
ly urged, 1st, That some diamonds, on being 
rubbed or chafed, are luminous in the dark. 
2 . That an electric spark, not larger, but 
much brighter, than the flame of a candle, 
may be produced, and yet that no part of the 
electric fluid is known to escape, in such a 
case, to distant places, but the whole pro- 
ceeds in the direction to which it is destined 
by the hand of the operator. We: ker or 
stronger sparks of this fluid are also known to 
differ in colour ; the strongest are white, and 
the weakest red, & c. 
To this opinion, however, there are many 
pressing, and, indeed, insurmountable objec- 
tions. 1st, The velocity of sound bears a 
very small proportion to that of light. Light 
travels, in the space of eight minutes, a dis- 
tance in which sound could not be commu- 
nicated in 17 years; and even our senses 
may convince us, if we attend to the explo- 
sion of gunpowder, &c. of the almost infinite 
velocity of the one compared with that of the 
other. 2dly, If light depended altogether 
on the vibrations of a fluid, no solid reason 
can be assigned why this fluid should cease 
to vibrate in the night, since the sun must 
always affect some part of the circumambient 
fluid, and produce a perpetual day. 3dly, 
The artifice of candles, lamps, &c. would be 
wholly unnecessary upon this hypothesis, 
since, by a quick motion of the hand, or of 
a machine contrived for this purpose, light 
might on all occasions be easily produced. 
4thly, Would not a ray of light, admitted 
through a small aperture, put in motion, ac- 
cording to this theory, the whole fluid con- 
tained in a chamber? In fact, we know that 
light is propagated only in right lines ; where- 
as sound, which depends upon vibration, is 
propagated in every direction. 5thly, The 
separation or extension of the rays, by means 
of the prism, can never be accounted for by 
the theory of a vibrating medium. 6thly, 
The texture of many bodies is actually 
changed by exposure to the light. The 
juice of a certain shell-fish contracts, it is 
well known, a very fine purple colour, when 
permitted to imbibe the rays of the sun ; and 
the stronger the light is, the more perfect the 
colour. Pieces of cloth wetted with this fluid 
become purple, even though inclosed in 
glass, if the solar light only is admitted ; but 
the effect is totally prevented by the inter- 
vention of the thinnest plates of metal, which 
exclude the light. Some of the preparations 
of silver, as luna cornea, will remain white 
if covered from the light, but contract a 
dark-purple coloOr when exposed to it ; and 
even the colour of plants is derived from the 
light, since a plant which vegetates in dark- 
less will be perfectly white. As colour is 
Yol. II. 
imparted by light, so it is also destroyed by 
it. It must have fallen within the observa- 
tion of every reader, that silks and other stuffs 
of delicate colours, are greatly affected by 
the action of light. Experiments have been 
made iipoin the same stuffs by exposing them 
to both li /-at and moisture in the dark, and 
also by exposing them to the light in the va- 
cuum of an air-pump, and it was found by all 
these experiments, that the change of colour 
was to be ascribed to the action of light. 
7 till v. With respect to the emission of light 
by diamonds and other stones, it is easily ac- 
counted for upon other principles; and the 
arguments founded upon the electric spark 
not being sensibly diminished, will meet with 
a satisfactory solution by considering the ex- 
treme rarity of light, and the minuteness of 
its particles. 
It is, therefore, almost universally agreed 
by the moderns, that light consists of a num- 
ber of extremely minute particles, Which are 
actually projected from the luminous body, 
and act by their projectile force upon the 
optic nerve. Concerning the nature of these 
particles, or rather of the matter of which 
they consist, there is less unanimity in the 
philosophical world. 
The first remarkable property of light is its 
amazing velocity. In the short space of one 
second a particle of light traverses an extent 
of 170,000 miles, which is so much swifter 
than the progress of a cannon-ball, that the 
light is enabled to pass a space in about eight 
minutes which could not be passed with the 
ordinary velocity of a cannon-ball in less 
than 32 years. The velocity of light is also 
found to be uniform, whether it is original, as 
from the sun, or reflected only, as from the 
planets. 
The mode of calculating the velocity of 
light is a branch of astronomy. It will suf- 
fice, therefore, in this place to remark, that 
by mathematical observations made upon the 
transits of Venus in 1761 and 1769, the dia- 
meter of the earth’s orbit was found to be 
about 163,636,800 geographical miles. When, 
therefore, the earth happens to be on that 
side of her orbit which is opposite to J upiter, 
an eclipse of his satellites, or any other ap- 
pearance in that planet, is observed to take 
place 15 or 16 minutes later than it would 
have done if the earth had been on that side 
of her orbit which is nearest to J upiter. E'rom 
the very accurate observations of Dr. Brad- 
ley, it appears, that the light of the sun passes 
from that luminary to the earth in eight mi- 
nutes and twelve seconds. 
The next property of light to which it is 
proper to advert is, that it is detached from 
every luminous or visible body in all direc- 
tions, and constantly moves in right lines. It 
is evident that the particles of light move 
continually in right lines, since they will not 
pass through a bended tube ; and since if a 
beam of light is in part intercepted by any 
intervening body, the shadow of that body 
will be bounded by right lines passing from 
the luminous body, and meeting the lines 
which terminate the interceding body. This 
being granted, it is obvious, that the rays of 
light must be emitted from luminous bodies 
in every direction ; since, whatever may be 
the distance at which a spectator is placed 
from any visible object, every point of the 
surface which is turned towards him is risible 
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lo him, which could not be upon any other 
principle. 
The rarity of light, and the minuteness o< 
its particles, are not less remarkable than ft*, 
velocity. If indeed flic Creator had not 
formed* its particles infinitely small, their ex 
cessive velocity would be destructive in the 
highest degree. Jt was demonstrated, that 
light moves about two millions of times as 
fast as a cannon-ball. The force with which 
moving bodies strike, is in proportion to their 
masses multiplied by their velocities; and 
consequently, if the particles of light were 
equal in bulk to the two-millionth part of a 
grain of sand, vve should be no more .able to 
endure their impulse than that of sand shot 
point-blank from the mouth of a cannon. 
1 lie minuteness of the rays of light is also 
demonstrable from the facility with which 
they penetrate glass, chrystal, and other solid 
bodies, which have their pores in a rectilinear 
direction, and that without the smallest di- 
minution of their velocity, as well as from the 
circumstance of their not being able to re- 
move the smallest particle of microscopic 
dust or matter which they encounter in their 
progress. A further proof might be added, 
that if a candle is lighted, and there is no 
obstacle to obstruct its rays, it will fill the 
whole space within two miles around it al- 
most instantaneously, and before it has lost 
the least sensible part of its substance. 
To the velocity with which the particles of 
light are known to move, may in a great mea- 
sure be attributed the extreme rarity and 
tenuity of that fluid. It is a well-known fact, 
that the effect of light upon the eye is not 
instantaneous, but continues for a consider- 
able time. Now we can scarcely conceive 
a more minute division of time than the 1 50th 
part of a second. If, therefore, one lucid 
point of the sun’s surface emits 150 particle* 
of light in one second, vve may conclude .that 
this will be sufficient to afford light to the eye 
without any seeming intermission ; and yet, 
such is the velocity with which light pro- 
ceeds, that still these particles will be at least 
1000 miles distant from each other. If it was 
not Indeed for this extreme tenuity of the fluid, 
it would be impossible that the particles 
should pass, as vve know they do, in all di- 
rections without interfering with each other. 
In all probability the splendour of all visible 
objects may be in proportion to the greater 
or less number of particles which are emitted 
or reflected from their surface in * given 
space of time ; and if we even suppose 309 
particles emitted successively from the sun’s 
surface in a single second, still these particles 
will follow each other at tlje immense distance 
of above 500 miles. 
Of the reflection qf light, or catoptrics. It 
has been already intimated, that the rays of 
light which proceed from any luminous body 
move always in straight lines, unless this di- 
rection or motion is changed by certain cir- 
cumstances ; and these are reflection, refrac- 
tion, and inflection. 
The great law of reflection, and which 
Serves to explain all its phenomena, is this, 
that the angle of reflection is always equal to 
the angle of incidence. It has been already 
intimated, that by the angle of incidence is 
meant the angle made by a ray of light with 
a perpendicular to the reflecting surface at 
the point where the ray falls; and by the 
angle of reflection, the ajpigle which the ray 
