TRANSACTIONS OF THE SECTIONS. 49 
to the solution of this difficulty, and at the last meeting of the Association at Oxford 
I mentioned the general fact, that when the eye had been for a short time closed 
and rested, and was then opened and directed to a moderately illuminated surface, a 
dark circular spot with a reddish brown penumbra, corresponding to the size of the 
foramen, was distinctly seen. The same effect was produced when the eye was not 
closed, but merely protected from light, which proved that the spot was not pro- 
duced by the act of closing the eye, or by any pressure of the eyeball on its socket. 
By various measurements of the diameter of this spot I found it to subtend an 
angle of about 4 degrees 35 minutes; and by taking the radius of curvature of the 
retina at0°5 of an inch, I found the diameter of the spot to be the ;';th of an inch, 
a result corresponding with sufficient accuracy with the measurement of the fora- 
men in the dead eye, as given by Soemmering, who makes it about the J>th of an inch. 
From this experiment it follows that when the eye is in its normal state, or in a state 
of rest, the choroid coat is less sensible to certain luminous impressions than the re- 
tina with the choroid coat behind it. S 
I now put the eye into an abnormal state, by exposing it for some time to a con- 
siderable degree of light, and upon repeating the preceding experiment, I saw upon 
the white ground a luminous spot, proving that when the eye was fatigued, or its 
sensibility diminished, the choroid coat was less affected than the re¢ina and choroid 
acting together. Between these two extreme conditions of the eye, namely, when 
the eye was neither in a state of rest or fatigue, no spot whatever appeared, the cho- 
_ roid cout alone and the retina and choroid coat acting together, being equally sen- 
sible to light. 
Anatomists have differed in opinion respecting the true form of the foramen cen- 
trale, Soemmering, the original discoverer of it, makes it circular, Some describe 
it as a fold in the retina, while others represent it as a double opening in the form 
of across. I have seen it myself in this latter form in the eye of a healthy person 
a few hours after death; but there can be no doubt that the process of removing 
the eye from its socket, and the pressure upon so tender a membrane as the retina 
by its separation from the membrane containing the vitreous humour, must alter the 
form of a circular foramen, shutting up the aperture and producing the appearance 
of a fold, or causing a double fold when it has the appearance of a cross. The 
roundness of the spot of variable sensibility I consider as establishing the true form 
of the foramen centrale, or of the limbus luteus which surrounds it, 
An Examination of Bishop Berkeley's “ New Theory of Vision.” 
By Sir Davin Brewster, KH, D.C.L., F.RS., § V.P.RS, Edin. 
The object of this paper was to examine the theory of Dr. Berkeley—the founda- 
tion of the Ideal Philosophy—in its optical relations. The author demonstrated (in 
opposition to the fundamental assumption of Dr. Berkeley *), “ that distance, both in 
monocular and binocular vision, is represented by lines on the retina.”” Hence every 
proposition of Dr. Berkeley’s founded on that erroneous assumption falls to the 
ground, But even if the fundamental proposition on which he rests his theory had 
been true, it would have been true only in vision with one eye, and therefore could 
not be applicable to human beings, whose vision is performed by two eyes. 
In support of his opinion that we see outness and distance directly by the eyes, and 
distinctly within a certain range of limited extent, while we judge of difference of 
distances beyond that range by various acquired means, the author described a 
number of experiments in binocular vision, where the eyes placed-the object at a 
fixed distance, which the nicest sense of touch, and the most accurate knowledge of 
the true place of the object, could not in the least degree influence ; and he supported 
his views by showing that the lower animals perceive distance at the instant of their 
birth, and that in every well-described case of the sudden restoration of sight in man 
by the extraction of the crystalline lens, or by the formation of an artificial pupil, 
owtness and distance were invariably seen. 
* “Tt is I think agreed by all, that distance of itself, and immediately cannot be seen ; for 
distance being a line directed endwise to the eye, it projects only one point on the fund of the 
eye; which point remains invariably the same, whether the distance be longer or shorter.” — 
An Essay, &e. § 1, 
1848. E ‘ 
