546 
OPT 
doftrine of light. We might here alfo mention Agui- 
lonius, a Jefuit of Bruflels, who publifhed a large trea- 
tife on optics in 1613. But Athanafius Kircher, who 
was one of the ableft philofophers and mathematicians of 
the age in which he lived, merits peculiar notice. He 
was about the fame age with Des Cartes, but outlived him 
thirty years. His large magnificent work, entitled “ Ars 
magna Lucis et Umbrae,” mult, at the time in which it 
is written, have been confidered as a very capital perform¬ 
ance ; and, though this author difcovered no new pro¬ 
perty of light, he contributed very much to the extenfion 
and improvement of the fcience of optics. Kircher took 
great pains in forming, from natural experiments, a table 
of the angles of refraftion, correfponding to any given 
angle of incidence, even to a minute of a degree, with re- 
fpeft to air and water ;-and he obferved the degrees with 
regard to wine, oil, and glafs. This table is contained in 
the work already cited. 
Scheiner made feveral important obfervations relating 
to vifion ; completing the difcovery, that vifion is per¬ 
formed by means of the images of external objefts upon 
the retina. By cutting away the coats of the back part of 
the eyes of Iheep and oxen, and prefenting feveral ob¬ 
jects before them within the ufual diftance of vifion, he 
faw their images diftinftly and beautifully painted upon 
the naked retina. He did the fame with the human eye ; 
and exhibited this curious experiment at Rome, in the 
year 1625. Scheiner particularly notices the correfpon- 
dence between the eye and the camera obfcura, and ex¬ 
plains a variety of methods to make the images of objefts 
ereft. As to the images of objefts being inverted in the 
eye, heacquiefces in the reafon for it affigned by Kepler; 
viz. that the mind traces the progrefs of the rays to the 
pupil, and refers them to that part of the objeft from 
which they, at that place, feemed to have proceeded. 
That the pupil of the eye is enlarged, in order to view re¬ 
mote objefts ; and, that it is contrafted while we are 
viewing thofe that are near, is a faft with which Scheiner 
was well acquainted, and which he proved by experi¬ 
ments, and illullrated by figures. Scheiner took confi- 
derable pains in afcertaining the denfity and refraftive 
power of all the humours of the eye ; and concludes, that 
the aqueous humour does not differ much from water in 
this refpect, nor the cryftalline from glafs, and that the 
vitreous humour is a medium between them both. He 
alfo traced the progrefs of the rays of light through all the 
humours of the eye; and, after difcuffing every hypothefis 
concerning the feat of vifion, demonftrates that it is the 
retina, fhowing that this was the opinion of Alhazen, 
Vitellio, Kepler, and all the moft eminent philofophers. 
After the publication of the Dioptrics of Des Cartes, in 
1637, a confiderable interval took place, during which 
Optics, and indeed fcience in general, made but little 
progrefs, till the “ Optica Promota” of James Gregory, 
ill 1663, feemed to put them again in motion. The au¬ 
thor of this work, a profound and inventive geometer, 
had applied diligently to the ftudy of optics and the im¬ 
provement of optical inltruments. The Optica PFomota 
embraced feveral new enquiries concerning the illumina¬ 
tion and diftinftnefs of the images formed in the foci of 
lenfes, and contained an account of the refleding- te- 
lejcope , Hill known by the name of its author. The con¬ 
federation which luggefted this inftrument was the imper* 
feftion of the images formed by fpherical lenfes, in con- 
fequence of which, they are not in plane, but in curved, 
l'urfaces. The defire of removing this imperfection led 
Gregory to fubllitute refleftion for refraction in the con- 
ftruftion of telefcopes; and, by this means, while he was 
feeking to remedy a fmall evil, he provided the means of 
avoiding a much greater one, with which he was not yet 
acquainted, viz. that which arifes from the unequal re- 
frangibility of light. The attention of the immortal 
Newton was about the fame time drawn to the fame ob¬ 
jeft, but with a perfeft knowledge of the defeft which he 
wanted to remove. Gregory thought it neceffary that 
I c s., 
the fpecula fltould be of a parabolic figure; and the exe¬ 
cution proved fo difficult, that the inftrument, during 
his own life, was never brought to any perfeftion : but 
the fpecula were afterwards conftrufted of the ordinary 
fpherical form ; and the Gregorian telefcope, till the time 
of Dr. Herfchel, was more in life than the Newtonian. 
Gregory was profeffor of mathematics at St. Andrew’s, 
and afterwards, for a fliort time, at Edinburgh. His 
writings ftrongly mark the imperfeft intercourle which 
fublifted at that time between this country and the con¬ 
tinent. Though the Optics of Des Cartes had been pub- 
lifhed twenty-five years, Gregory had not heard of the dif¬ 
covery of the law of refraftion, and had found it out only 
by his own efforts; happy in being able, by the fertility 
of his genius, to fupply the defefts of an infulated and 
remote fituation. 
A courfe of Leftures on Optics, delivered at Cambridge 
in 1668, by Dr. Barrow, and publifhed in the year fol¬ 
lowing, treated of all the more difficult queftions which 
had occurred in that ftate of the fcience, with the acute- 
nefs and depth which are found in all the writings of that 
geometer. This work contains fome new views in optics, 
and a great deal of profound mathematical difcuflion. 
About this time, Grimaldi, a learned Jefuit, the com¬ 
panion of Riccioli in his aftronomical labours, made 
known fome optical phenomena which had hitherto ef- 
caped obfervation. They refpefted the adtion of bodies 
on light; and, when compared with refleftion and refrac¬ 
tion, might be called, in the language of Bacon’s philo- 
fophy, crepttfcular inftances, indicating an aftion of the 
fame kind, but much weaker and lei's perceptible. Hav¬ 
ing ftretched a hair acrofs a fun-beam admitted through 
a hole in the window-fhutter of a dark chamber, he was 
furprifed to find the ffiadow much larger than the natural 
divergence of the rays could have led him to expeft. 
Other fafts of the fame kind made known the general law 
of the diffradion or inflexion of light; and fliowed that 
the rays are afted on by bodies, and turned out of their 
reftilineal courfe, even when notin contaft, but at arnea- 
fureable diftance from the furfaces or edges of fuch bodies. 
Grimaldi gave an account of thofe fafts in a treatife 
printed at Bologna in 1665. 
Optics, as indeed all the branches of natural philofo- 
pliy, have great obligations to Huygens. . The former 
was among the firft fcientific objefts which occupied his 
mind ; and his “ Dioptrics,” though a polthumous work, 
is moft of it the compolition of his early youth. It is 
written with great perfpicuity and precilion, and is faid 
to have been a favourite book with Newton himfelf. 
Though beginning from the firft elements, it contains a 
full development of the matters of greateft difficulty in 
the conftruftion of telefcopes, particularly in what con¬ 
cerns the indiftinftnefs arifing from the imperfedt foci 
into which rays are united by fpherical lenfes; and rules 
are deduced for conftrufting telefcopes, which, though 
of different fizes, ihall have the fame degree of diftinft¬ 
nefs, illumination, &c. Huygens was, belides, a praftical 
optician; he polilhed lenfes, and conftrufted telefcopes, 
with his own hands, and fome of his objeft-glaffes were 
of the enormous focal diftance of 130 feet. To his Diop¬ 
trics is added a valuable treatife De Formandis Vi Iris. 
The controverfy concerning the nature of light was 
revived at this period by the hypothefis of Des Cartes ; 
who maintained that light is neither a fubltance, as Epi¬ 
curus and fome others had fuppofed, nor yet a mere pro¬ 
perty of bodies, as Ariftotle conceived ; but the motion of 
a fubtle fluid, communicated inftantaneoufly by the pref- 
fure of a luminous body. Intlead of the perfeftly-folid 
globules, in the motion of which Des Cartes thought 
that light confiftedjMalebranchefubftituted fluid vortices, 
and fuppofed that every impreffion communicated to any 
one of them, is immediately tranfmitted to thofe that are 
contiguous to it; fo that the propagation of light is limi- 
lar to that of found. The later Cartefiansin general fup¬ 
pofed, that the fluid by which light is tranfmitted, is elal- 
