564 
OPTICS. 
primitive forms of the cryftals and the number of their 
axes. Thus we find, that, when the primitive form is a 
cube, or regular odtohedron, the cry ft a! poffelfes three 
axes; when it is a right quadrangular priftn with a rec¬ 
tangular bafe, the number of axes is two; the number is 
alfo two, when the cryftal is an odtohedron in which the 
pyramids have a rectangular or a rhomboidal bafe; and, 
when there is-only one axis, the primitive forms are either 
hexabeural prifms, rhomboids with obtufe fummits, or 
odtohedrons in which the pyramids have fquare bafes. 
We have no doubt that future experiments will ftill fur¬ 
ther generalize thefe appearances, and eftablilh an arrange¬ 
ment of clafles among cryftallized fubftances, and their 
optical properties, which will greatly affift the mineralo- 
gift in afcertaining the primitive forms of many doubtful 
cryftals, and in reconciling thofe anomalies which fre¬ 
quently perplex us in the mineral kingdom. Thomfons 
Annals of Philofophy for Apr. 1814. and Jan. 1816. Edin¬ 
burgh Review for July 1819. 
Difcoveries concerning Vijion. 
Maurolycus was the firft who demonftrated that the 
cryftaliine humour of the eye is a lens which coliedts the 
light Bluing from external objedts, and converges them 
upon the retina. He C.id not, however, feem to be aware, 
that an image of every vifible objedt was thus formed 
upon the retina, though this feems hardly to have been a 
ftep beyond the difeovery' he had made. Montucla con¬ 
jectures, that he was prevented from mentioning this 
part of the difeovery, by the difficulty of accounting for 
the upright appearance of objedts. This difeovery was 
made by Kepler; but he, too, was much puzzled with 
the inverfion of the image upon the retina. “ The rec¬ 
tification of thefe images,” he fays, “is the bufinefs of the 
mind; which, when it perceives an impreflion on the 
lower part of the retina, confiders it as made by rays pro¬ 
ceeding from the higher parts of objedts; tracing the 
rays back to the pupil, where they crofs one another.” 
This is the true explanation of the difficulty ; and is 
exadtly the fame as that which was lately given by Dr. 
Reid. 
The fubjedt of vifion occupied the attention of Des 
Cartes, fie explains the methods of judging of the mag¬ 
nitudes, fituations, and diftances, of objedts, by the di- 
redfion of the optic axes; comparing it to a blind man’s 
judging of the fize and diftance of an objedt by feeling it 
with two fticks of a known length, when the hands in 
which lie-holds them are at a known diftance from each 
other. He alfo remarks, that, having been accu*ftomed to 
judge of the fituation of objedts by their images falling 
on a particular part of the eye, if, by any diftortion of the 
eye, they fall 011 a different place, we are apt to miftake 
their fituation, or imagine one objedt to be two, in the 
ltame way as we imagine one (tick to be two, when it is 
placed between two contiguous fingers laid acrofs one 
another. The diredtion of the optic axes, lie fays, will 
not ferve us beyond 15 or 20 feet, and thechange ofform 
of the cryftaliine not more than three or four feet. For 
ho imagined, that the eye conforms itfelf to different dif- 
tances by a change in the curvature of the cryftaliine, 
which he fuppofed to be a mufcle, the tendons of it being 
the ciliary precedes. In another place he fays, that the 
cjiunge in the conformation of the eye is of no ufe to us 
for the purpofe of judging of cliftances beyond four or 
five feet, and the angle of the optic axes not more than 
100 or 200 feet: for this reafon, he fays, that the fun 
and moon are conceived to be much more nearly of the 
fame fize than they are in reality. White and luminous 
objedts, he obferves, appear larger than others, and alfo 
the parts contiguous to thofe on which the rays adtualiy 
impinge ; and, for the fame reafon, if the objects be fmail, 
and placed at a great diftance, they will alw'ays appear 
round, the figure of the angles difappearing. 
The celebrated Dr. Berkeley, bithop of Cloyne, pub- 
liflied, in 1709, An Eflay towards a New Theory oDVifion, 
4 
in which he folves many difficulties. He does not admit 
that it is by means of thofe lines and angles, which are 
ufeful in explaining the theory of optics, that different 
diftances are eftimated by the fenfe of fight; neither does 
he think that the mere diredtion of the optic axes, or the 
-greater or lefs divergency of the rays of light, are fufficient 
for this purpofe. “ I appeal (fays he) to experience, whe¬ 
ther any one computes its diftance by the bignefs of the 
angle made by the meeting of the two optic axes ; or whe¬ 
ther he ever thinks of the greater or lefs divergency of 
the rays which arrive from any point to his pupil. Nay, 
whether it be not perfedtly impofiable for him to perceive, 
by fenfe, the various angles wherewith the rays, according 
to their greater or lefs divergency, fell upon his eye.” 
That there is a neceffary connexion between thefe various 
angles, &c. and different degree.s of diftance, and that 
this connexion is known to every perfon (killed in optics, 
he readily acknowledges; but “.n vain _(he obferves) 
(hall mathematicians tell me, that I perceive certain lines 
and angles, which introduce into my mind the various 
notions of diftance, fo long as I am confcious of no fuch 
thing.” He maintains, that diftance, magnitude, and 
even figure, are the objedts of immediate perception only 
by the fenfe of touch ; and that, when we judge of them 
by fight, it is from different fenfations felt in the eye, 
which experience has taught us to be the confequence of 
viewing objedts of greater or lefs magnitude, of different 
figures, and at different diftances, Thefe fenfations, with 
therefpedtive diftances, figures, and magnitudes, by ..which 
they are occafioned, become fo clofely affociated in. the 
mind, long before the period of diftindt recolledtion, that 
the prefence of the one inftantly fuggefts the other; and 
we attribute to the fenfe of fight, thofe notions wdaich 
are acquired by the fenfe of touch, and of which certain 
vij'ual fenfations are merely the figns or fymbols, jufc as 
words are the fymbols of ideas. Upon thefe principles, he 
accounts for fingle and eredf vifion. Subfequent writers 
have made confiderable difcoveries in the theory of vifion; 
and, among them, there is hardly any one to whom this 
branch of lcience is fo much indebted as to Dr. Reid, and 
Dr. Welis. 
As an image of every vifible objedt is painted on the re¬ 
tina of each of our eyes, it thence becomes a natural 
queltion, Why we do not fee every thing double ? It was 
the opinion of fir Ifaac Newton and others, that objedts 
appear fingle, becaufe the two optic nerves unite before they 
reach the brain. But Dr. Porterfield fiiows, from theobfer- 
vation of leveral anatomifts, that the optic nerves do not 
mix or confound their fubftance, being only united by. a 
clofe cohefion ; and objedts have appeared fingle where the 
optic nerves were found to be disjoined. 
Dr. Briggs fuppofed that fingle vifion was owing to the 
equal tendon of the correfponding parts of the optic 
nerves, whereby they vibrated in a lynchronous manner. 
But, betides feveral improbable circumftances in this 
account, Dr. Porterfield ftiows, that facts do by no mean's 
favour it. To account for this phenomenon, therefore, 
this ingenious writer fuppofes, that, by an original law 
in our natures, we imagine objedts to be iituated foins- 
wbere in a right line drawn from the pidture of it upon 
the retina, through the centre of the pupil. Confs- 
quently, the lame objedt appearing to both eyes to be in 
the fame, place, the mind cannot diftinguifti it into two. 
In anfvver to an o'ojedtion to this hypothefis, from objects 
appearing double when one eye is diftorted, he fays the 
mind miftakes the pofition or the eye, imagining that it 
had moved in a manner correfponding to the other; in 
which cafe the conciufion would have been juft. 
This principle, however, has been thought fufficient to 
account for this appearance. Originally, every objedt, 
making two pidtures, is imagined to be double; but, by 
degrees, we find, that, when two correfponding parts of 
tiie retina are irnpreifed, the objedt is but one ; but, if 
thofe correfponding' parts be changed by the diftortion 
of one of the eyes, the objedt muft again appear double, 
as 
