OPTICS. 
lei, and move on in the line on, oo: but the 
concave lens, being made of hint glass, and 
upon a shorter radius, has a greater refrac- 
tive power, and therefore they diverge a little 
alter they come out of it ; and if no third 
lens was interposed, they would proceed di- 
verging in the lines opt , opt ; but, by the in- 
terposition of the third lens ovo, they are 
again made ;o converge, and meet in a focus 
somewhat more distant than the former, as 
at x. By the concave lens the violet rays are 
also refracted, and made to diverge : but, 
having a greater degree of refrangibility, the 
same power of refraction makes them diverge 
somewhat m ire than the red ones ; and thus, 
if no tim'd lens was interposed, they would 
proceed in such lines as hnn, Imn. As the 
tliilerentl v-coloored rays then fail upon the 
third ien's with different degrees of diver- 
gence, it is plain that the same power of re- 
faction in that lens will operate. upon them in 
such a manner as to bring them all together 
to a focus very nearly attiie same point. The 
red rays, it is true, require the greatest 
power of refraction to br.ng them to a focus; 
but they fall upon liie lens \\ ;th the least de- 
gree of divergence. I'he violet rays, though 
they require the least power of refraction, 
yet have the greatest degree of divergence ; 
and thus all meet together at the point x, or 
very nearly so. It was afterwards demon- 
strated by M. Zelker of Petersburgh, that it 
is the lead used in the composition of the 
crown glass, which gives it this remarkable 
property of dispersing the extreme rays ; and 
he found that this property w as increased in 
proportion to the quantity of minium, or red 
lead, which was employed in the manufacture 
of the glass. 
The more we investigate the works of na- 
ture, the greater reason have we to admire 
the v isdom of its author, and that wonderful 
adaptation of oar organs, in the minuter par- 
ticulars, to the general laws which pervade 
the universe. The subject before us affords 
a striking instance to corroborate this remark. 
We have hitherto supposed the eye to be a 
lens capable only of enlargingand contracting, 
and consequently, from the description now 
given of the rays of light, it must be incapa- 
ble of obviating the confusion which must 
arise from their different degrees of refrangi- 
bility. But here the use of that wonderful 
structure of parts, and the different fluids in 
the eye, is clearly seen. M he eye is, in fact, 
a compound lens. Each fluid has its proper 
degree of refrangible power. The shape of 
the lenses is altered at will, according to the 
distance of tiie object ; and the three sub- 
stances having the proper powers of refrangi- 
bility, the effects of an achromatic glass are 
without difficulty produced by the eye, 
whose mechanical structure and exact ar- 
rangement ot substances it is in vain for the 
art of man to imitate. 
From what lias , been stated, the principal 
phenomena of colours may, without much 
difficulty, be explained. 
If all the different-coloured rays which the 
prism affords are reunited in the focus of a 
convex lens, the produce will be white ; yet 
these same rays, which, taken together, form 
white, give, after the point of their reunion, 
that is, beyond the point where tiiey cross 
each other, tne same colours as those which 
departed from the prism, but in a reversed 
order, by the crossing of the rays: the reason 
of which is clear ; for the ray being white be- 
fore it was divided by the prism, must ne- 
cessarily become so by the reunion ot its 
parts, which the difference of refrangibility 
had separated, and this reunion cannot ifi any 
mariner tend to alter or destroy the nature of 
the colours ; it follows then that they must 
appear again beyond the point of crossing. 
A similar effect will be produced, if the dis- 
persed rays are received. from, the prism upon 
a concave reflector. In the locus of the re- 
flector they will unite and form a white cr 
colourless image of the sun. But it is cu- 
rious to remark, that if any ore of the colours 
is stopped in its progress to the reflector by 
the interposition of a wire, or any other slen- 
der opaque body, then the image in the focus 
will be an imperfect white, or a mixed co- 
lour. Beyond the focus the rays separate 
again, as in the case of their passing through 
a convex lens, and form the coloured spec- 
trum, only the order of the colours from the 
crossing of the rays is inverted. 
In the same manner, if we mix a certain 
proportion of red colour with orange, yellow, 
green, blue, indigo, and violet, a colour will 
be produced which resembles that which is 
made by mixing a little black with white, 
and which would be entirely white if some of 
the rays were not lost or absorbed by the 
grossness of the colouring matter. 
A colour nearly approaching to white, is 
also formed by colouring a piece of round 
pasteboard with the different prismatic co- 
lours, and causing it to be turned round so 
rapidly, that no particular colour can be per- 
ceived. 
If to a single ray of the sun, divided by 
the prism, which will then form an oblong 
coloured spectrum, a thick glass deeply co- 
loured with one of the primitive colours is 
applied, for example red, the light which 
passes through will appear red only, and will 
form a round image. 
The component rays of light may be sepa- 
rated by other means than by the prism. It is a 
common amusement of children to blow round 
bubbles of soap, dissolved in water, from the 
bowl of a tobacco-pipe ; and these bubbles 
will, in the sunshine, commonly exhibit most 
of the prismatic colours. Indeed the same 
tiling may be at any time observed in the 
bubbles made by agitating soap and water. 
As these bubbles are thin vesicles ol the mat- 
ter dissolved in the fluid, they are commonly 
supposed to vary in their thickness, and to act 
in this w'ay in separating the rays. If two 
pieces of glass, also of an unequal surface, are 
gently pressed together, round the point ot 
contact circles of different colours will be 
formed. Sir Isaac Newton employed for 
this experiment the object-glasses ot two te- 
lescopes of a long focus, which it is well 
known are much less convex than the com- 
mon spectacle-glasses. One was a plano- 
convex for a telescope of 14 feet, and the 
other a double-convex for one ot 50 feet. 
Upon pressing the glasses close together, at 
the point of contact circles ol coloured light 
appeared, and they increased in number and 
si/.e as the pressure was increased. I he or- 
der of the colours next to the point in con- 
j tact, w hicn was black, w as blue, yellow, 
| white, yellow, and red. W ithout this circle 
j another appeared, consisting of violet, blue. 
309 
green, yellow, and red. A third succeeded 
of purple, blue, green, yellow, and red ; and 
a fourth of green and red. r ! he outer circles 
were paler, and more obscure, than those 
within. _ . , r 
The appearance of these circles is ue.i- 
neated in flg. 15. where a, b, c, d, t , 
f, g, h, i, k ; /, in, n, o, p ; q, ry s, t ; u, 
x ; y, z ; denote the colours in order mom 
the centre, namely, black, blue, green, yellow, 
red ; purple, blue, green, yellow, red ; green, 
red: greenish blue, red; greenish blue ; red- 
dish white. 
Various theories have been offered to ac- 
count for this separation of the rays, but none 
of them are quite satisfactory. Perhaps it 
Mr. Deleval's experiments on transmitted 
and reflected light were carefully pursued* , 
phenomenon. 
If two thick glasses, the one red and die 
other green, are placed one upon another, 
they will produce a perfect opacity, though 
each of them, taken separately, is transpa- - 
rent; because the one permits the red rays 
only to pass through it, and the othei only 
green ones ; therefore when these two glasses 
are united, neither of those kind of rays can 
reach the eye ; because the first permits only 
red rays to pass, and green ones are the only 
raws which the second can transmit. 
If the rays of the sun are made to fall yeiy 
obliquely upon the interior surface oi a prism, 
the violet-coloured rays wall be reflected, ana 
the red, &c. will be transmitted ; if the obli- 
quity of incidence is augmented the blue will 
be also reflected, and the other transmitted ; 
the reason of which is, that the rays which 
have the most refrangibility are also tlio^e 
which are the easiest reflected. 
In whatever manner we examine the co- 
lour of a single prismatic ray, we shall al- 
ways find, that neither refraction, reflection, 
nor any other means, can make it forego its 
natural hue ; but if we examine the artificial 
colouring of bodies by a microscope, it will 
appear a rude heap of colours, unequally 
mixed. If we mix a blue and yellow to 
make a common green, it will appear mode- - 
rate!: beautiful to the naked eye; but when 
we regard it with microscopic attention, it 
seems a confused mass of yellow and blue 
parts, each particle reflecting but one sepa- 
rate colour. 
Of the rainbozv , and other remarkable phe- 
nomena of light . — Since the rays of light are 
found to be decompounded by refracting >ur- 
faces, we can no longer be surprised at the 
changes produced in any object by the inter- 
vention of another. T he vivid colours which 
gild the rising or the setting sun, must ne- 
cessarily differ from those which adorn its 
noon-dav splendour. There must be the 
greatest variety which the liveliest fancy can 
imagine. The clouds will assume the most 
fantastic forms, or will lour with the darkest 
hues, according to the different rays which . 
are reflected to our eves, or the quantity ab- 
j sorbed by the vapours in the air. Theigno- 
rant mu. titude will necessarily be alarmed by 
! the sights in the heavens ; by the appearance • 
at one time of three, at another of five, - uns ; . 
of circles of various magnitudes round the.- 
sun or. moon ; and thence conceive that some - 
iata change must taive place in the physical 1 
or the moral world, some fall of dnpoe. or 
tremendous earthquake : while the optician 
