OPTICS. 
2Q6 
or the incident light might pass through it. 
Behind this hole, in the second board, he also 
placed a prism, so that the light, after passing 
both the boards, might suiter a second re- 
traction before it reached the wall. He then 
moved the lirst prism in such a manner as to 
make the several parts of the image cast upon 
the second board pass successively through 
the hole in it, that he might observe to what 
places on the wall the second prism would 
refract them. The consequence was, that 
the coloured light, which formed one end of 
the image, suffered a refraction considerably 
greater than that at the other end ; in other 
words, rays or particles of light of one colour 
were found to be more refrangible than those 
ot another. The true cause, therefore, of 
the length of the image was evident ; since it 
was proved by the experiment, that light 
was not homogeneal, but consisted of differ- 
ent particles or rays, which were capable of 
different degrees of refrangibility, according 
to which they were transmitted through the 
prism to the opposite wall. It was further 
evident from these experiments, that as the 
rays of light differ in refrangibility, so they 
also differ in exhibiting particular colours, 
some rays producing the colour red, others 
that of yellow, blue, &c. and of these differ- 
ent-coloured rays, separated by means of 
the prism according to their different de- 
grees of refrangibility, the oblong figure on 
the wall was composed. But to relate the 
great variety of experiments, by which he 
demonstrated these principles, or the exten- 
sive application of them, would lead us too 
much into detail; let it suffice to say, that he 
applied his principles to the satisfactory ex- 
planation of the colours of natural bodies, of 
the rainbow, and of most of the phenomena 
of nature where light and colour are con- 
cerned; and that almost every thing which 
we at present know upon these subjects was 
laid open by his experiments. 
His observations on the different refrac- 
tive powers of different substances are cu- 
rious and profound ; but chemistry was at 
that period scarcely in a state sufficiently ad- 
vanced to warrant all his conclusions. The 
general result is, that all bodies seem to have 
their refractive powers proportional to their 
densities, excepting so far as they partake 
more or less of inflammable or oily par- 
ticles. 
; ; 'A>l»ti*lhcovery of the different refrangibility 
of the component rays of light suggested de- 
fects in kh,e construction of telescopes, which 
were before uqt bought of, and in the creative 
hand af a Newton led to some no less extra- 
ordinary improvements in them. It is evi- 
dent, that since the rays of light are of differ- 
ent refrangibilhieSj the more refrangible will 
converge to aftfyicus much sooner than the 
less reftangil)he,'qpiii?equently that the whole 
beam cannot be'. brought to a focus in any 
one point,; *so ’that the focus of every object- 
glass will, be a circular space of considerable 
diameter, namely, about one fifty-fifth of the 
aperture of the telescope. To remedy this, 
he adopted Gregory’s idea of a reflector, with 
such improvements as have been the basis of 
all the present instruments of this kind. 
When a science has, been carried to a cer- 
tain degree of perfection, subsequent disco- 
veries are too apt to be considered as of little 
importance. The real philosopher will not, 
however, regard the discoveries on light and 
colours, since the time of Newton, as unwor 
thy his attention. By a mere accident, a 
very extraordinary property in some bodies 
of imbibing light, and afterwards emitting it 
in the dark, was observed. A shoemaker of 
Bologna, being in quest of some chemical 
secret, calcined, among other things, some 
stones of a particular kind, which he found 
at the bottom of Mount I’eteius ; and casually 
observed, that when these stones were car- 
ried into a dark place, after having been ex- 
posed to the light, they possessed a sell-illu- 
minating power. Accident afterwards disco- 
vered the same property in other substances. 
Baldwin, of Misnia, dissolving chalk in aqua- 
fortis, found that the residuum, after distilla- 
tion, exactly resembled the Bolognian stone 
in retaining and emitting light, whence it 
now lias the name of Baldwin’s phosphorus; 
and M. Du hay observed the same property 
in all substances that could be reduced to a 
calx by burning only, or after solution in ni- 
trous acid. These facts seem to establish the 
materiality of light. 
Some very accurate calculations were 
made about the year 1725 by Dr. Bradley, 
which afforded a more convincing proof of 
the velocity of light, and the motion of the 
earth in its orbit. Nor must we forget M. 
Bouguer’s very curious and accurate expe- 
riments for ascertaining the quantity of light 
which was lost by reflection, the most deci- 
sive of which was by admitting into a darken- 
ed chamber two rays of light, one of which 
he contrived should be reflected, and the 
other fall direct on the opposite wall ; then 
by comparing the size of the apertures by 
which the light was admitted (that through 
which the direct ray proceeded being much 
smaller than that through which the reflected 
ray was suffered to pass, and the illumination 
on the wall being equal in both), he was en- 
abled to form an exact estimate of the quan- 
tity of light which was lost. To prove the 
same effect with candles, he placed himself in 
a room perfectly dark, with a hook in his hand, 
and having a candle lighted in the next room, 
he had it brought nearer to him till he could 
just see the letters, which were then 24 feet 
from the candle. He then received the light 
of the candle reflected by a looking-glass 
upon the book, and he found the whole dis- 
tance of the book from the source of the light 
(including the distance from the book- to the 
looking-glass) to be only 15 feet; whence he 
concluded, that the quantity of direct light 
is to that of reflected as 576 to 225 ; and si- 
milar methods were pursued by him for mea- 
suring the proportions of light in general. 
The speculations of Mr. Melville concern- 
ing the blue shadows which appear from 
opaque bodies in the morning and evening, 
when the atmosphere is serene, are far from 
uninteresting. These phenomena he attri- 
butes to the power which the atmosphere 
possesses’ of reflecting the fainter and more 
refrangible rays of light, the blue, violet, Ac. 
and upon this principle he also explained 
the blue colour of the sky, and some other 
phenomena. 
The same period produced Mr. Dollond’s 
great improvement in the construction of 
telescopes. It consists in using three glasses 
of different refractive powers, crown and flint 
glass, which correct each other. The great 
dispersion of the rays which the flint-glass 
produces, is the effect of the lead, and is in 
proportion to the quantity of .that metal 
which is used in its composition. Mr. Mar- 
tin found the refractive powers of different 
glasses to be in proportion to their specific 
gravity. 
Several discoveries and improvements 
have been made since the time ©f Newton 
iu that branch of optics which relates more 
immediately to vision. One of these is not 
only curious in itself, but led to the explana- 
tion of several circumstances relating to vi- 
sion. M. De la Motte, a physician of Dant- 
zick, was endeavouring to verify an experi- 
ment of Scheiner, in which a distant object 
appeared multipl ed when viewed through 
several holes made with the point of a pin in 
a card, not further distant from one another 
than the diameter of the pupil of the eve; 
but notwithstanding all his labour, he was 
unable to succeed, till a friend happening to 
call upon him, he desired him to make the 
trial, and it answered perfectly. This friend 
was short-sighted ; and when he applied a 
concave glass dose to the card, the object, 
which seemed multiplied before, now ap- 
peared but one. 
The last, though not least, successful ad- 
venturer in this branch of science, is Mr, 
Delaval, who, in a paper read before the 
Philosophical Society of Manchester, in 1734, 
has endeavoured, with great ingenuity, to ex- 
plain the permanent colours of opaque bo- 
dies. The majority of those philosophers, 
who have treated of light and colours, have, 
he observes, supposed that certain bodies or 
surfaces reflected only one kind of ravs, and 
therefore exhibited the phenomena of co- 
lours; on the contrary, Mr, Delaval, by a 
variety of well-conducted experiments, evin- 
ced, that colours are exhibited, not’ by re- 
flected, but by transmitted light. '1 Ills he 
proved by covering coloured glasses and 
other transparent coloured media, on the 
further surface, with some substance perfect- 
ly opaque, when lie found they reflected no 
colour, but appeared perfectly black, lie 
concludes, therefore, as the fibres or bases of 
all vegetable, mineral, and animal substances, 
are found, when cleared of heterogeneous 
matters, to be perfectly white ; that the rays 
of light are in tact reflected from these white 
particles, through coloured media, with 
which they are covered; that these media 
serve to intercept and impede certain rays in 
their passage through them, while, a free pas- 
sage being left to others, they exhibit, ac- 
cording to these circumstances, different 
colours. This he illustrates by the fact re- 
marked by Dr. Halley, who, in diving deep 
into the sea, found that the upper part of his 
hand, when extended into the water from 
the diving-bell, reflected a deep-red colour, 
w hile the under part appeared perfectly 
green. The conclusion is, that the more 
refrangible rays were Intercepted and reflect- 
ed by particles contained in the sea-water, 
and were consequently reflected back by the 
under part of the hand; while the redrays, 
which were permitted to pass through the 
water, were in the same manner reflected by 
the upper part of the hand, which therefore 
appeared of a red rose-colour. Those me- 
dia, our author thinks, transmit coloured 
light with the greatest strength, which have 
the strongest refractive power. 
Of the nature of li g/; t \ Numerous opi- 
nions have successively .been adopted con- 
