í
I i .
60 O f the e y e s in F I S H E S .
The whole cryilalline lens of the ox, v
The whole cryilalline lens of the cod,
The outer part of the cryftaUine lens of tlie ox,
The outer part of the cryilalline lens of the cod,
The nucleus of the cryilalline lens of the ox,
The nucleus of the cryilalline lens of the cod,
Parti.
I 1 04
Í 1 6 5
1 0 7 0
I 140
I 160
1200
On comparing the cryilalline lens of tlie ox with that of the cod, I found
that they were very nearly of the ihape delineated in Table XL*, fig. i . where
the outermoil dotted lines reprefent an horizontal feilion of the lens of the ox,
and the continued lines the fame of the cod: and they meafured one-fourth part
of the dianjeter reprefented; or one-tenth of an inch in this figure reprefents
one-fortieth of an inch in their cryilallines: Or I found the radii of the fpheres
which compofe the lens in thefe two animals to be in fortieths of an inch nearly
as in the following table.
The radius of the anterior part of the lens of the ox, . . 21
The radius of the anterior part of the lens of the cod, . . 14
The radius of the poilerior part of the lens of the ox, . . 15
The radius of the poilerior part of the cod, which is nearly an hemifplieref,
In the laft place, I found that the focus of the rays of the fun, which was
nearly ¿th part of an inch in diameter, was diilant from their poilerior part.
In the ox, ~ of an inch;
But in the cod, not more than
And I obfcrved that the diilance of the focus from the furface of tiae whole lens,
or from that of "the nucleus in. the cod, was nearly the fame.
The focus of tlie nucleus of the lens-of^-tke cod placed in water was about ^
of an incii diftant from its back part.
Hence it is evident, that the cryilalline lens of the cod, but efpecially its
nucleus, is much more denfe, and refradls light more readily, than that of the
ox: nay, that its refraction, inilead of bearing proportion merely to its fpecific
weight, with fome little addition on account of its containing inflammable
matter, which is tlie common fuppofition i , is more powerful than that of
common
t The celebrateJ Dr Petit has, in the Mem. de I'Acad. 1730, p. 9. defcribed the breadth and tliicitnefs of tlie cryllal-
Une lens, particubrly of tlie ox, and the radii of the fpheres which compofe it, with fo much appearance of accuracy,
t!iat his defcriptions have been univcrfaJIy received by authors: Yet I have not only obfervcd by meafuring, that the lens of
tJie ox is much more convex on both fides than he rcprefcnts'it; but it will be found impolTible, with fucli radii, to dcfcribe
a lens of tiic breailtli and thicknds he himfelf ailigns to it. When we aflume his radii and breadth, a much fiattci'
lens than that of the ox is produced; and which the innermoil dotted lines I ALm reprefent in Tali.XL *. fig. i.
Nay, it is demonIIrable, that lie has erred greatly in every example in Iiis Table of the human lens, as well as in that of
the lens of the ox, his radii not corrcfponding, in a finglc inftance, with the breadth and tliicknefs of tlie lens he found by
X Dr Porterficid, v. i. p. 232. 5 p. p. 278. See. Pemberton. Kaller, EI. Ph. 1. 16. p. 402- Í
rcfraclikin Parvjm elTe qua atjuara fuperat prxrogativam tiupcri fatenlur. *
O f the E Y E S in E I S H E S. 6 t
common glafs: for the nucleus of the lens of the cod, which is nearly fpherical,
forms its focus at the diilance of a little more than one-fixth part of its diameter;
whereas parallel rays of light, paifing from air through a fphere of glafs, are
not collcded into a focus till they are one-fourth of its diameter diilant from its
poilerior part.
The lateft and moil eminent writers on the eye have taught, that a principal
ufe of the fpherical figure of the cryilalline humour of fiüies, is to enable it to
colleél the rays of light more powerfully, or nearer to the fore part of tlie eycy
than could be done by a lens compofed of two fmall portions of a fphere 'j'.
But on cailing the eye on TableXL*, fig. 2. it appears, that the focus of a
fphere is nearly, but not exadly, for the proportion varies according to the medium,
as much more diilant from its anterior part, than the focus of a- lens
compofed of two portions of the fame fphere, by as much as tlic tliicknefs or
diameter of the fphere Ts longer than the tliicknefs or axis of the lens.
Nay, from the fame figure it is evident, that a lens, the axis or thicknefs of
which is equal to the radius, and its breadth equal to tlie dianieü . of tlie fphere,
and compofed dierefore of two portions of a larger fphere, has its focus cohfiderably
nearer to its fore part tlian the fphere has.
Upon the whole, therefore, we are led to the conclufion. That the primary
ufe of the almoil completely fpherical figure of tlie cryilalline lens of fiilies, or
great convexity efpecially of the anterior part of tlieir lens, which I find projects
in tlie cod about feven fortieths of an inch beyond the iris, is to take in
a large field of the objeéls around them; which was particularly neceíTary, as the
motion of their neck is inconfiderable.
T o enable them, with the fame length of die axis of the eye as In the quadruped,
to collect into a focus on tlie retina the rays of light coming from the
denfe medium of water, four cliief circumflanccs concur.
In the firil place. We obfcrve that rlieir cryilalline lens is more convex, or
compofed of porrions of fmaller fpheres, dian in land-animals.
In the next place, Wc have found that their cryilalline lens is, in correfponding
parts, much more denfe than in animals which live In air.
Thirdly, That the lens in fiflies poiTeiTes powers of refrading light far beyond
what have been calculated by authors, who have proceeded on the fuppofition
that thefe powers were proportioned nearly to Its fpecific gravity.
In the laft placc, The vitreous humour of fiihes being lighter than that of landanimals,
the rays of light iiTuIng from dieir lens will be refraded in a greater
degree, or brought fooner to a focus p
Q . I
. p. 4Ö8. Hinc pifcibus,t Dr Povtcrficld on tlic Eyes, v. ii. p. 261, 262. Dr Haller, El. Phyf. L. s quorum ladii n
IIus refringuntur, lens pcnc fphxrica.
} Dr Ilaller, on this fubjea, Pr. Lin. Phyf. DXXXVIII. and in Elcm. Phyf. t. v. p. 467. commits tlic ovcrfight of
fuppofmg, that bccaufe the vitreous iiuroour is more rare than the cryftaUine lens, it will have the effea of making the
rays longer of coming to a focus i whereas tlic lefs its denfity is fuppofcd in proportion to that of the lens, the more the
rays will be rcfrnftcd and the diilance of the focus lelTencd. Hence the vitreous humour is made vifcid and Iieavy,
merely becaufc a cuihion for fupporting tlic lens at a proper dillancc from tlie retina was necefTary.
I , I
I'flT!