In order to ascertain this point, upon which the whole controversy may be said to turn, several experiments were performed with great care upon an individual who had lost the crystalline, in order to ascertain how far he re tained the power of adjustment ; but easy as it might ap pear to come to a conclusion on this subject, great diffi culties occurred, and nothing decisive was accomplished ; Dr. Young conceiving that the trials which were made confirmed his view of the subject ; while Sir E. Home adopted a contrary opinion, and supposed that the adjust ment was effected by the external muscles of the eye pressing upon the sides of the ball, and thus increasing the distance from the cornea to the retina. But this hy-.
pothesis would seem to be disproved by a subsequent ex periment of Dr. Young's, in which the eye was immersed in water, through which the sight was directed, when it did not appear that the power of adjustment was, in any degree, impaired by this means ; yet here it is obvious, that no change in the external figure of the cornea could produce any effect, but that the change must consist in an alteration of the relative position of the internal parts of the eye. Upon the whole, therefore, we consider the opinion of Dr. Young as the most probable ; but we do not think that the muscularity of the crystalline has been sufficiently established.
The next order of parts in the eye are those composed of nervous matter, by which a sensation of the image is produced, is transmitted to the brain, and constitutes a perception. This is accomplished by means of the retina and the optic nerve. The retina is an expansion of nervous matter, which lines the posterior part of the membranes enclosing the humours, and which receives the image formed by the rays that enter the cornea. From the ana logy of the nervous system generally, and from the con nexion of this part with the optic nerve, it has generally been considered as the immediate seat of vision. This opinion was, however, controverted about a century ago by many physiologists, in consequence of the discovery of Marriotte, that the part of the retina, which lies over the commencement of the optic nerve, is not sensible to light. It was argued, that because in this part, which is insensible, the retina is present, while the choroid is not so, the choroid and not the retina must be the seat of vi sion ; and, as the sensibility of membrane was a favourite doctrine of the Stahlians, the hypothesis of Matiotte gain ed many followers. The fact, indeed, as discovered by him, is generally admitted ; but the conclusion is so re pugnant to the other analogies of the animal economy as to be now universally discarded. The inference which we should draw from it is, that there is a difference in the functions of nervous matter, according as it is in the form of a thin expansion, or a dense cord, the first being adapted for receiving impressions, the latter for transmit ting them to the brain.
The third order of parts in the eye is very numerous, and contains many curious and elaborate structures. One of the most important of these is the iris, the coloured ring which surrounds the pupil, or aperture through which the rays are admitted into the interior of the eye. The use of the iris is, to regulate the quantity of light which enters the pupil. For this purpose it has the power of contracting in a bright light, and thus excluding part of the rays which would otherwise enter the eye ; on the contrary, in a feeble light, it expands so as to admit a greater number of rays. Physiologists were, for a long time, unable to decide upon the nature of the mechanism by which this effect is brought about ; but it is now proved that the iris is a mucular part, and that its fibres contract when they are exposed to the stimulus of light ; this opinion is supported by the fact, that certain individuals have a voluntary power over this organ. Fon tana has made an important observation concerning it, that the light does not act directly upon the iris, but upon the retina ; for he found, that when a small beam was made to fall upon a part of the iris, no contraction ensued, but that the effect was produced when it was directed through the pupil, so as to fall upon the retina.
After this account of the form and structure of the eye, and the uses of its principal parts, we must inquire into the manner by which the sensation of sight is produced. The image of the object being formed upon the retina, a conception of it is transmitted to the brain, and the object of our investigation will be to ascertain in what manner this is effected, or more generally what change takes place in the nerve and the brain, which immediately precedes mental impressions. Two hypotheses have been advanced on this subject : the first, and the one most generally known, is that of animal spirits ; the other that of vibra tions. Acccrding to the first, it was supposed that the brain is a species of gland, which secretes a subtile fluid, termed the animal spirits. The nerves were conceived to be tubes, which convey these spirits to all parts of the body, and when an impression is made upon the extremity of a nerve, the animal spirits arc so affected as to trans mit the impression to the brain. The ancients had an opinion somewhat resembling this. It was very gene rally adopted at the revival of letters, and still maintains its ground ; yet it appears to be entirely destitute of di rect evidence ; it is difficult to form any distinct concep tion of it ; and it does not correspond with the structure of the part concerned, or with the usual analogies of the ani mal economy.