OPTICS. 
d'jced the refults of his experiments into the form of a 
table, exhibiting; the angles of refraction correfponding to 
the angles of incidence, which he had tried in water and 
glafs. It was not, however, till long after this'period, (as 
Montucla hasobferved,) that the law which connects thefe 
angles was difcovered. The caufe of refraction appeared 
to him to be the refiitance which the rays fufferdn palling 
into the denfer medium of water or glafs ; and one can fee 
in his reafoningan obfcure idea of the refolution of forces. 
He alfo treats of the rainbow ; and fhows that refraction, 
as well as reflection, is neceflary to form it: he defines the 
colours of the rainbow to~be three only, and makes many 
other obfervations on its phenomena. In his account of 
the horizontal moon, he agrees with Alhazen ; obferving, 
that, in the horizon, the flcy feems to touch the earth, 
and appears much more diftant from us thin iu the zenith, 
on account of the intermediate fpace containing a greater 
variety of objeCts upon the viiible furface of the earth. 
Ten years after the publication of Vitellio’s work, 
Peccam, archbishop of Canterbury, wrote a treatife of 
dircCl' optics, which was then called perfpe£live; but 
without making any addition to the exilting flock of 
optical knowledge. His treatife is faid to be concife and 
judicious ; and to contain, among other things, a very 
clear and diftinct account of the reafon why the Iky near 
the horizon appears mere diftant from us than at any other 
plate. 
Contemporary with Vitellio and Peccam, was Roger 
Bacon, diftinguilhed for purfuing the path of true phi- 
lofophy in the midft of an age of ignorance and error. 
He applied to the ftudy of optics with peculiar diligence : 
it does not appear, however, that he added much to the 
difeoyeries of Alhazen and Ptolemy, with whole writings, 
particularly thole of the former, he feems to have been 
well acquainted. In fome things he was much be¬ 
hind the Arabian optician, as he fuppofed, with the an¬ 
cients, that viflon is performed by rays emitted from the 
eye. It nvuft however be allowed, that the arguments 
employed on both Tides of this queftion are fo weak and in- 
conclufive, as very much to diminilh the merit of being' 
right, and the demerit of being wrong. What is 
moft to the credit of Bacon, is the near approach he 
appears to have made to the knowledge of lenfes, 
and their ufe in ifiifting viflon. Alhazen had remarked, 
that finall objeCts, letters for inftance, viewed through 
a fegment of ;t glafs fphere, were leen magnified, and 
that it is the larger fegment which magnifies the moft. 
The fpherical fegment was fuppofed to be laid with 
its bale on the letters, or Other minute objeCts which 
were to be viewed. Bacon recommends the fmaller feg¬ 
ment ; and obferves, that the greater, though it magnify 
more, places the objefc farther off than its natural pofi- 
tion, while the other brings it nearer. This (hows fuffi- 
ciently, that he knew how to trace the progrefs of the 
rays of light through a fpherical tranfparent body, and 
underftood, what was the thing lealt obvious, how to 
determine the place of the image. Smith, in his Optics, 
endeavours to (how, that thefe concluflons were purely 
theoretical ; and that Roger Bacon had never made any 
experiments with fuch glafles, notwithftanding that he 
fpeaks as if he had done fo. This fevere remark proceeds 
on fome flight inaccuracy in Bacon’s defeription, which, 
however, does not leem fuflicient to authorife fo harfh a 
conclulion. The probability appears rather to be, as 
Molineux fuppofed, that Bacon had made experiments 
with fuch glafles, and was both praCticaliy and theoreti¬ 
cally acquainted with their properties. At the fame time, 
it muft be acknowledged, that his credulity on many 
points, and his fondnefs for the marvellous, which, with 
every refpeCt for his talents, it is impoffible to deny, take 
foinething away from the force of his teftimony, except 
when it is very exprefsly given. However that may be 
in the prefent cafe, it is probable that the knowledge of the 
true properties of thefe glafles, whether it was theoretical 
or practical, may have had a (hare in introducing the ufe 
513 
of lenfes, and in the invention of fpe&acles, which took 
place not long after. It would certainly be deArable to 
afeertain the exaCt period of an invention of fuch lingular 
utility as this; one that diffufesits advantages fo widely, 
and that contributes fo much to the folace and comfort of 
old age, by protecting the moft intellectual of the fenfes 
againft the general progrefs of decay. In the obfeurity 
of a dark age, careiefs about recording difeoveries *of 
which it knew not the principle or the value, a few faint 
traces and imperfect indications ferve only to point out 
certain limits-within which the thing fought for is con¬ 
tained. Seeking for the origin of a difeovery is like leek- 
ing for the fource of a river, where innumerable ftreams 
have claims to the honour, between which it is impoffible 
to decide 5 and where the only thing that can be known 
with certainty, is the boundary by which they are 
all circumfcribed. The reader will find the evidence 
concerning the invention of fpeCtacles very fully difeufied 
in Smith’s Optics, vol. ii. from which the moft probable 
conclulion is, that the date goes back to the year 1313, 
and cannot with any certainty be traced farther. 
The iapfe of more than two hundred years brings us 
down to Maurolycus, and to an age when men of Science 
ceafed to be fo thinly fcattered over the waftes of time. 
Maurolycus, belides great knowledge of the pure mathe¬ 
matics, was diftinguilhed for his (kill in optics. He was 
acquainted with the eryftalline lens, and conceived that 
its office is to trail Unit to the optic nerve the Jpecies of ex¬ 
ternal objeCts; and, in this procefs, he does not confider 
the retina as any way concerned. This theory, though 
fo imperfect, led him, neverthelefs, to form a right judg¬ 
ment of the defeats of fliort-lighted and long-lighted eyes. 
In one of his rirft works, Theoremata de Laniine et Umbra , 
publilhed in 1375, he alfo gives an accurate folution of a 
queftion propofed by Ariltode, viz. why the light of the 
fun, admitted through a finall hole, and received on a 
plane at a certain diftance from it, always illuminates a 
round fpace, whatever be the figure of the hole itfelf; 
whereas, through a large aperture, the illuminated fpace 
has the figure of the aperture. To conceive the reafon of 
this, fnppofe that the figure of the hole is a triangle ; it is 
plain that at each angle the illuminated fpace will be ter¬ 
minated by a circular arch, of which the centre correfponds 
to the angular point, and the radius to the angle fub- 
tended by the fun-’s femidiameter. Thus the illuminated 
fpace is rounded oft" at the angles; and, when the hole is fo 
fmall that the lize of thofe roundings bears a large pro¬ 
portion to the diftance of their centres, the figure comes 
near to a circle, and may be to appearance quite round. 
This is the true folution, and the fame with that of 
Maurolycus. The fame author appears alfo to have ob- 
ferved the cauftic curve formed by reflexion from a con¬ 
cave lpeculum. 
A confiderable ftep in optical difeovery was made at 
this time by Baptifta Porta, a Neapolitan, who invented 
the camera abjeura, about the year 1560, and defcribed it 
in a work, entitled “ Magia Naturalis.” The light was 
admitted through a fmall hole in the window-lhutter of 
a dark room, and gave an inverted picture of the objects 
from which it proceeded, on the oppolice wall. A lens 
was not employed in the firft conftruCtion of this appa¬ 
ratus, but was afterwards tiled ; and Porta went fo far as 
to confider how the efteCt might be produced without in- 
verfion. He appears to have been a man of great inge¬ 
nuity; and, though much of the Magia Naturalis is di¬ 
rected to frivolous objeCts, ic indicates a great familiarity 
with experiment and observation. It is remarkable, that 
we find mention made in it of the reflection of cold by a 
Speculum; an experiment which, of late, has drawn fo 
much attention, and has been fuppofed to be fo entirely 
new. The cold was perceived by making the focus fail on 
the eye, which, in the abfence of the thermometer, was 
perhaps the belt meafure of fmall variations of tempera¬ 
ture. Porta’s book was extremely popular; and, when 
we find it quickly tranflated into Italian, French, Spani/h, 
a and 
