556 OPT 
of them ; and they were all much more diilimfl when the 
paper was held obliquely. Thefe coloured ftreaks, like 
thofe on the outfide of the lhadow, were bent in an arch, 
round the acute angles of the lhadow, as they are repre- 
fented in fig. 9. At this angle alfo, as at D, other fiiorter 
lucid ftreaks were vifible, bent in the form of a plume, as 
they are drawn betwixt D and C, eacli bending round and 
meeting agairi in D. Thefe angular ftreaks appeared, 
though the plate or rod was not wholly immerled in the 
beam of light, but the angle of it only; and they in- 
creafed in number with the breadth of the plate. If the 
plate was very thin, the coloured ftrealcs bent round from 
the oppofite fides, and met one another as at B. 
In order to obtain a more fatisfaffory proof, that rays 
of light really bend in palling by the edges of bodies, 
he admitted a beam of light into a dark room, as before ; 
and, at a great diftance from it, he fixed a plate EF, fig. 10. 
with a final) aperture GH, which admitted only a part of 
tlie beam of light; and found that, when the light trans¬ 
mitted through this plate was received at fome diftance 
upon a white paper, the bafe IK was confiderably larger 
than it could poftibly have been made by rays iffuing in 
right lines through the two apertures. Grimaldi gene¬ 
rally made the aperture CD or part of a foot, and 
the fecond aperture, GH, or ; and the diftances 
DG and GN were at lealt n feet. The obfervation 
■was made about mid-day in the fummer-time, when the 
atmofphere was free from all vapours. 
Grimaldi alfo made the fame experiment that has been 
recited from Dr. Hooke, in which two beams of light, en¬ 
tering a dark room by two fmall apertures near one an¬ 
other, proje6ied cones of light, which, at a certain dif¬ 
tance, in part coincided ; and he particularly obferved, 
that the dark boundaries of each of them were vifible 
within the lucid ground of the other. 
To thefe dilcoveries of Grimaldi, we fliall fubjoin an 
additional obfervation of Dechales ; who found, that, if 
a piece of poliflied metal, with fmall fcratches in it, be 
expcfed to the beams of the fun in a darkened room, it 
will reflect the rays ftreaked with colours in the direction 
of the fcratches ; as will appear, if the remedied light be 
received upon a piece of white paper. That thefe colours 
are not produced by refraflion, he fays, is manifeft ; for, 
if the fcratches be made upon glafs, the effect will be the 
fame ; and in this cafe, if the light had been refrafted at 
the furface of the glafs, it.would have been tranfmitted 
through it. From theie and many other oblervations, he 
concluded, that colour does not depend upon the refpac- 
tion'of light only, nor upon a variety of other circum- 
ftarfces, which he particularly enumerates, but upon the 
intenfity of the light only. 
The experiments of Grimaldi and Hooke were repeated 
and extended by fir Ifaac Newton, and were in fome mea- 
fure explained by that diftinguilhed philofopher. He 
made in a piece of lead a fmall hole, the 4-zd part of an 
inch in diameter. Through this hole he let into his dark 
chamber a beam of the fun’s light; and found, that the 
Ihadows of hairs, and other flender fubftances, placed in 
it, were confiderably broader than they would have been 
if the rays of light had palled by thofe bodies in right 
lines. He therefore concluded, that they mufthave palfed 
as they are reprefented in fig. n. in which X reprefents a 
feftion of the hair, and AD, BE, &c. rays of light palling 
by at different diftances, and then falling upon the wall 
GQ. Since, when the paper which receives the rays is at 
a great diftance from the hair, the lhadow is broad, it mult 
follow, that the hair a£ts upon the rays at fome confuler- 
able diftance from it, the aftion being ftrongeft on thofe 
rays which are at the lealt diftance, and growing weaker 
and weaker on thofe which are farther off, as is repre¬ 
fented in this figure ; and hence it comes to pafs that the 
lhadow of the hair is much broader, in proportion to the 
diftance of the paper from the hair, when it is nearer than 
when it is at a greater diftance. 
By wetting a polilhed plate of glafs, and laying the hair 
ICS. 
in the water upon the glals, and then laying anpther po¬ 
lilhed plate of glafs upon it, fo that the water might fill 
up the fpace between the glaffes, he found that the lhadow 
at the fame diftance was as big as before; fo that this 
breadth of lhadow mull: proceed from fome other caufe 
than the refradlion of the air. 
The ihadows of all bodies placed in this light were bor¬ 
dered with three parallel fringes of coloured light, of 
which that which was neareft to the lhadow was the 
broadeft and moft luminous, while that which was fartheft 
from it was the narroweft, and fo faint as to be fcarcely 
vifible. It was difficult to diftinguilh thefe colours, un- 
lefs when the light fell very obliquely upon fome fmooth 
white body, fo as to make them appear much broader 
than they would otherwife have done; but, in thefe cir- 
cumftances, the colours were plainly vifible, and in the 
following order: The firft or innermoft fringe was vio¬ 
let, and deep blue next the lhadow ; light-blue, green, 
and yellow, in the middle ; and red without. The fecond 
fringe waS almoft contiguous to the firft, and the third to 
the lecond ; and both were blue within, and yellow and 
red without; but their colours were very faint, efpecially 
thofe of the third. The colours, therefore, proceeded in 
the following order from the lhadow : violet, indigo, pale- 
blue, green, yellow, red ; blue, yellow, red; pale-blue, 
pale-yellow, and red. The Ihadows, made by fcratches, 
and bubbles in polilhed plates of glafs, were bordered with 
the like fringes of coloured light. Meafuring thefe fringes 
and their intervals with the greateft accuracy, he found 
the former to be in the progreffion of the numbers x, 
Vb \/\, and their intervals to be in the fame progreffion 
with them, that is, the fringes and their intervals toge¬ 
ther to be nearly in continual progreflion of the numbers, 
1,1/1, Vb VbVb 
Having made the aperture £ of an inch in diameter, and 
admitted the light as formerly, fir Ifaac placed, at the 
diftance of two or three feet from the hole, a llieet of pafte- 
board, black on both fides; and in the middle of it he 
made a hole about of an inch fquare, and behind the 
hole he faftened to the pafteboard the blade of a lharp 
knife, to intercept fome part of the light which paffed 
through the hole. The planes of the pafteboard and 
blade of the knife were parallel to each other, and per¬ 
pendicular to the rays ; and, when’ they were 1b placed, 
that none of the light fell on the pafteboard, but all of it 
palled through the hole to the knife, and there part of it 
fell upon the blade of the knife, and part of it paffed by 
its edge, he let that part of the light which palled fall on 
a white paper 2 or 3 feet beyond the knife, and there he 
faw two ftreams of faint light llioot out both ways from, 
the beam of light into the lhadow.. But, becaule the fun’s 
direct light, by its brightnefs upon the paper, obfcured 
thefe faint ftreams, fo that he could fcarcely fee them, he 
made a little hole in the midft of the paper for that light 
to pafs through and fall on a black cloth behind it; and 
then he faw the two ftreams plainly. They were fimilar to 
one another, and pretty nearly equal in length, breadth, 
and quantity of light. Their light, at that end which 
was next to the fan’s direft light, was pretty itrong for 
the fpace about J of an inch or \ an inch, and gradually 
decreafed till it became infenlible. 
The whole length of either of thefe ftreams, meafured 
upon the paper at the diftance of 3 feet, from the knife, 
was about 6 or 8 inches; fo that it fubtended an angle, 
at the edge of the knife, of about 10 or 12, or at moft 14, 
degrees. Yet fometimes he thought he law it llioot 3 or 
4 degrees farther; but with a light fo very faint, that he 
could hardly perceive it. This light he fufpecteef might, 
in part at leaft, arife from fome other caufe than the two 
ftreams. For,.placing his eye in that light, beyond the 
end of that ftream which was behind the knife, and look¬ 
ing towards the knife, he could lee aline of light upon 
its edge ; and that, not only when his eye was in the line 
of the ftreams, but alfo when it was out of that line, ei¬ 
ther towards tlie point of.the knife, or towards the handle. 
This, 
