OPTICS. 
rately taken, is extremely irregular, fome of them really 
concave, and others convex ; but, in reducing them to 
a middle Hate, they are to be regarded as planes. Never- 
thelefs, he confiders them as planes only with refpett to 
the reception of the rays; for, as they are almoft all 
curves, and as, befides this, many of thofe whofe fitua- 
tion is different from others contribute to the fame eft’efts, 
the rays always iffue from an aftual or imaginary focus, 
and, after reflection, always diverge from another. 
Difcoveries concerning the Inflexion of Light. 
This property of light was not difcovered till about 
the middle of the feventeenth century. The perfon who 
firft made the difcovery was father Grimaldi; at leaft, he 
firft publifhed an account of it, in his treatife De Lumine , 
Coloribus, et Iride , printed in 1666. Dr. Hooke, how¬ 
ever, laid claim to the fame difcovery, though he did not 
make his obfervations public till fix years after Grimaldi. 
Dr. Hooke, having darkened his room, admitted a 
beam of the fun’s light through a very fmall hole in a 
brafs plate. This beam, fpreading itfelf, formed a cone, 
the vertex of which was in the hole, and the bafe was on 
a paper fo placed as to receive it at fome diftance. In 
the image of the fun, thus painted on the paper, he ob- 
ferved that the middle was much brighter than the edges, 
and that there was a kind of dark penumbra about it, of 
about a 16th part of the diameter of the circle; which 
he afcribed to a property of light that he promifed to ex¬ 
plain. Having obferved this, at the diftance of about 
two inches from the former, he let in another cone of 
light; and, receiving the bafes of them at fuch a diftance 
from the holes that the circles interfered each other, he 
obferved that there was not only a darker ring, encom- 
pafting the lighter circle, but a manifeft dark line, or 
circle, as in fig. 4. which appeared even where the limb 
of the one interfered with that of the other. 
In the light thus admitted, he held an opaque body BB, 
fig. 5. fo as to intercept the light that entered at a hole in 
the window-fliutter O, and was received on the fcreen AP. 
In thefe circumftances, he obferved, that the lhadow of the 
opaque body (which was a round piece of wood, not 
bright or polilhed) was fometimes enlightened all over, 
but more efpecially towards the edge. In order to fliow 
that this light was not produced by reflection, he admit¬ 
ted the light through a hole burnt in a piece of pafteboard, 
and intercepted it with a razor which had a very Iharp 
edge ; but ftill the appearances were the very fame as be¬ 
fore ; fo that he concluded that they were occafioned by 
fome new property of light. He diverfified this experi¬ 
ment, by placing the razor fo as to divide the cone of 
light into two parts, and placing the paper fo that none 
of the enlightened part of the circle fell upon it, but 
only the lhadow of the razor ; and, to his great furprife, 
he obferved what he calls a very brijk and viflhle radiation 
ftriking down upon the paper, of the fame breadth with 
the diameter of the lucid circle. This radiation always 
ftruck perpendicularly from the line of lhadow, and, like 
the tail of a comet, extended more than ten times the 
breadth of the remaining part of the circle. He found, 
wherever there was a 'part of the interpofed body higher 
than the reft, that, oppofite to it, the radiation of light 
into the lhadow was brighter, as in the figure; and, 
wherever there was a notch or gap in it, there would be a 
dark ftroke in the half-enlightened lhadow. From all 
thefe appearances, he concluded, that there is a deflection 
of light, differing both from reflection and refraction, and 
feeming to depend on the unequal denfity of the confti- 
tuent parts of the ray, whereby the light is dilperfed from 
the place of condenfation, and rarefied, or gradually di¬ 
verged, into a quadrant; that this deflection is made to¬ 
wards the fuperficies of the opaque body perpendicularly; 
that thofe parts of the diverged radiations which are de¬ 
flected by the greateft angle from the ftraight or direct ra¬ 
diations are the fainteft, and thofe that are deflected by 
the leaft angles are the ftrongeft; that rays cutting each 
555 
other in one common aperture do not make the angles at 
the vertex equal; that colours may be made without re¬ 
fraction ; that the diameter of the fun cannot be truly 
taken with common fights; that the fame rays of light, 
falling upon the fame point of an objeCt, will turn into 
all forts of colours, by the various inclinations of the ob¬ 
ject ; and that colours begin to appear when two pulfes 
of light are blended fo well, and fo near together, that 
the fenfe takes them for one. 
Grimaldi’s experiments were as follows.—Having in¬ 
troduced a ray of light through a very fmall hole, AE, 
fig. 6. into a darkened room, he obferved that the light 
was diffufed in the form of a cone,-the bafe of which was 
CD ; and that_, if any opaque body, EF, was placed in 
this cone of light, at a confiderable diftance from the 
hole, and the lhadow received upon a piece of white paper, 
the boundaries of it were not confined within GH, or the 
penumbra IL, occafioned by the light proceeding from 
different parts of the aperture, and of the difk of the fun, 
but extended to MN. At this he was very much fur- 
prifed, as he found that it was broader than it ought to 
have been made by rays pafling in right lines by the edges 
of the objeCt. But the moll remarkable circumftance’in 
this appearance was, that, upon the lucid part of the bafe, 
CM and ND, ftreaks of coloured light were plainly diftin- 
guilhed, each being terminated by blue on the fide next 
the lhadow, and by red on the other; and, though thefe 
coloured ftreaks depended, in fome meafure, on the lize 
of the aperture AB, becaufe they could not be made to 
appear if it was large, yet he found that they were not 
limited either by it, or by the diameter of the fun’s difk. 
He farther obferved, that thefe coloured ftreaks were not 
all of the fame breadth, but grew narrower as they re¬ 
ceded from the lhadow, and were each of them broader 
the farther the lhadow was received from the opaque 
body, and alfo the more obliquely the paper on which 
they were received was held with refpeCt to it. He never 
obferved more than three of thefe ftreaks. 
To give a clearer idea of thefe coloured ftreaks, he drew 
the reprefentation of them exhibited in fig. 7. in which 
NMO reprefents the largeft and molt luminous ftreak, 
next to the dark lhadow X. In the fpace in which M is 
placed there was no diftinCtion of colour ; but the fpace 
NN was blue, and the fpace OO, on the other fide of it, 
was red. The fecond ftreak QPR, was narrower than the 
former ; and of the three parts of which it confifted, the 
fpace P had no particular colour, but QQ was a faint blue, 
and RR a faint red. The third ftreak, TSV, was exaCtly 
fimilar to the tw r o others, but narrower than either of 
them, and the colour ftill fainter. Thefe coloured ftreaks. 
he obferved to lie parallel to the lhadow of the opaque 
body; but, when it was of an angular form, they did not 
make the fame acute angles, but were bent into a curve, 
the outermoft being rounder than thofe that were next 
the lhadow, as is reprefented in fig. 8. If it was an in¬ 
ward angle, as DCH, the coloured ftreaks, parallel to 
each other of the two lides, crofted without obliterating 
one another; only the colours were thus rendered either 
more intenfe or mixed. 
Within the lhadow itfelf, Grimaldi fometimes perceived 
coloured ftreaks, fimilar to thofe above mentioned on the 
outfide of the lhadow. Sometimes he faw more of them, 
and fometimes fewer ; bur, for this purpofe, it was necef- 
fary to have ftrong light, and to make the opaque body 
long and moderately broad. A hair, for inftance, or a 
fine needle, did not anfwer fo well as a thin and narrow 
plate ; and the ftreaks were moll diltinguilhable when the 
lhadow was taken at the greateft diftance; though the 
light grew fainter in the fame proportion. The numbers 
of thefe ftreaks increafed with the breadth of the plate. 
They were at leaft two ; and fometimes four, if a thicker 
plate were made ufe of. But, with the fame plate, more 
or fewer ftreaks appeared, in proportion to the diftance at 
which the lhadow was received; but they were broader 
when they were few, and narrower when there were more 
of 
