55 ? 
OPT 
This line of light appeared contiguous to the edge of the 
knife, and was narrower than the light of theinnennoft 
fringe, and narrowed: when his eye was fartheft-from the 
direct light; and therefore feemed to pafs between the 
light of that fringe and the edge of the knife 5 and that 
which parted neareft the edge feemed to be molt bent. 
He then placed another-knife by the former, fo that 
their edges might be parallel, and look towards one ano¬ 
ther, and that the beam of light might fall upon both 
the knives, and fome part of it pafs between their edges. 
In this fituation he obferved, that when the diftance of 
then' edges was about the 400th of an inch, the ftream di¬ 
vided in the middle, and left a Ihadow between'the two 
parts. ’ This Ihadow was fo dark, that all the light which 
palled between the knives feemed to be bent to the one 
hand or the other; and, as the knives ftill approached 
each other, the Ihadow grew broader, and the ftreams 
Ihorter, next to it, till, upon the contact of the knives, all 
the light vanilhed. Hence fir Ifaac concluded, that the 
light which is leaf!: bent, and which goes to the inward 
ends of the ftreams, pafles by the edges of the knives at the 
greateft diftance; and this diftance, when the Ihadow be¬ 
gan to appear between the ftreams, was about the 800th • 
of an inch ; and the light which palled by the edges of the 
knives at diftances ftill lefs and lefs, was more and more 
faint, and went to thofe parts of the ftreams which were 
farther from the direCt light; becaufe, when the knives 
approached one another till they touched, thofe parts of 
the ftream vaniflted laft which were fartheft from the di- 
red! line. 
In the experiment of one knife only, the coloured fringes 
did not appear; but, on account of the breadth of the 
hole in the window, became fo broad as to run into one 
another, and, by joining, to make one continual light 
in the beginning of the ftreams ; but, in the laft experi¬ 
ment, as the knives approached one another, a little be¬ 
fore the Iliadow appeared between the two ftreams, the 
fringes began to appear on the inner ends of the ftreams, 
on either fide of the direCt light; three on one fide, made 
by the edge of one knife, and three on the other fide, made 
by the edge of the other knife. They were the moft dif- 
tindl when the knives were placed at the greateft diftance 
from the hole in the window, and became ftill more dif- 
tinCt by making the hole lefs; fo that he could fometimes 
fee a faint trace of a fourth fringe beyond the three above- 
mentioned ; and, as the knives approached one another, 
the fringes grew more diftinCt and larger, till they va- 
niftied ; the outermoft vaniftiing firft, and the innennoft 
laft. After they were all vanilhed, and the line of light 
in the middle between them was grown very broad, ex¬ 
tending itfelf on both fides into the ftreams of light de- 
fcribed before, the above-mentioned Ihadow began to ap¬ 
pear in the middle of this line, and to divide it along the 
middle into two lines of light, and increafed till all the 
light vanilhed. This enlargement of the fringes was fo 
great, that the rays which went to the innennoft fringe 
leemed to be bent about 20 times more when the fringe 
was ready to vanifh, than when one of the knives was 
taken away. 
From both thefe experiments Newton concluded, that 
the light of the firft fringe palled by the edge of the knife 
at a diftance greater than the 800th of an inch ; that the 
light of the fecond fringe paffed by the edge of the knife 
at a greater diftance than the light of the firft fringe, and 
that of the third at a greater diftance than that of the fe¬ 
cond ; and that the light of which the ftreams above- 
mentioned confifted, paffed by the edges of the knives at 
lefs diftances than that of any of the fringes. 
Difccveries concerning the Polar if at ion of Light, 
It has been remarked, that, with the exception of fir 
Ifaac Newton’s brilliant difcoveries, the exertions of fci- 
entific men have done lefs for optics than for almoft any 
other branch of phylical fcience; and, with thofe difco¬ 
veries, the progress of folid improvement feemed, till 
Vox.. XVII, No. 1 j97. 
I c s. 
within thefe few years, to have begun and ended. Nor 
was this becaufe the attention of philofopliers had been 
lefs directed to this fcience than to others. On the con¬ 
trary, few branches of philofo.phy- have had greater men 
devoted to their inveftigation ; and it was difcouraging 
enough to think, that, while the. general improvements 
in mathematics had brought ftatics, dynamics, and aftro- 
nomy, to a very high ftate of perfection, and even reduced 
the complicated and perplexing doCtrine of chances to 
certainty, the phenomena of light alone, remained im¬ 
perfectly explained, and apparent!}' beyond the reach of 
human calculations. The extreme difficulty of fubjefting 
the properties of light to precife examination, and of 
eftablilhinggeneral laws by which the > might be governed, 
rendered the refearches of philofopliers for the moft part 
unavailing; and the fruits of their labours were little 
more than a mafs of experiments, from which little fatif- 
faCtory or conclufive could be obtained. 
Among all the phenomena in optics, none had puzzled 
philofopliers more than refraction. Of thofe who fuc- 
ceeded Des Cartes and Fermat, fome attempted to explain 
the law of refraCtion, like the latter, by the doCtrine of 
final caufes. Suppofing that the refracted ray would pro¬ 
ceed from one medium to another in the Ihorteft time, 
Leibnitz conceived that the ray would choofe the track 
which was leaft difficult; the difficulty of the path being 
meafured by a ratio compounded of its length and of the 
refiftance of the medium. Among thefe fanciful and ar¬ 
bitrary hypothefes, that of Huygens (who, though ad¬ 
mirable as a geometer and natural philofopher, neverthe- 
lefs retained many prejudices of the Cartefian fchool) is 
more deferving of notice, as it led him to form an hypo- 
thefis for explaining the more extraordinary phenomena 
of double refraction, which, though vague indeed as any 
of thofe originating on the principles of Des Cartes, has 
yet been found to reconcile, in a fingular manner, certain 
anomalous refults obtained by our own countryman Dr. 
Woollafton ; and afterwards, ftill further to coincide with 
the experiments of Malus. We have noticed, that Huy- 
ens fuppofed light to confift of certain undulations of a 
uid, highly elaltic and fubtle, of extreme tenuity, and 
capable of expanding, in a circular direction, with great 
rapidity, round a luminous centre. Befides this, he con¬ 
ceived each of the circular waves to confift of an infinite 
number of particular undulations, whole centres were 
diffufed through every part of the expanded fluid, and 
which all together united in forming the principal one. 
The ordinates of the principal undulation were conceived 
to depend upon the rapidity of thofe which formed it; 
fo that if, by any means, the velocities of their expan- 
fions varied, the direction of the ordinates was changed ; 
and, in the variation of thefe ordinates, the refraction 
confifted. Thus, where a ray was incident obliquely 
upon a medium which was- more difficult to penetrate, 
that is, which was denfer than the furrounding medium, 
he fuppofed it to move more flowly ; confequently, the 
expanlions extended with lefs rapidity, the direction of 
the ray was changed, and the variation which enfued in 
the ordinates, or the fines of incidence and refraction, 
were proportional to the refiftance the light met with in 
penetrating the medium. Now, in the cafe of double re¬ 
fraCtion ; when the ray entered a doubly-refraCting me¬ 
dium, fuch as Icelandfpar, Huygens conceived each un¬ 
dulation would affume the form of an oblate fpheroid, 
owing to the internal conftruCtion of the cryftal : the 
centre of this fpheroid was the point of incidence, its 
axis parallel to the Ihorter diagonal of the rhomboid, and 
bearing to the perpendicular diameter the ratio of about 
9 to 10. The extraordinary refraCtion, lie conceived to 
depend upon the ordinate of the generating ellipfe, ex¬ 
actly in the fame way as the ordinary refraft ion wai made 
to depend upon the ordinate of the circle. 
Wild and fanciful as this hypothefis feems, and altoge¬ 
ther repugnant to every rational principle of induction, 
we cannot be furprifed that it foon funk into negleCt 1 and 
7 C it 
