The glass, combination is a perfect ophthalmoscope. again, has been replaced with advantage by a mirror, generally a concave mirror, with an aperture in the centre, through which the observer looks. Fig. 22 shows the method of using this apparatus constructed after Ruete's plan. The light is placed near the person under observation, A. The rays emitted fall upon the concave mirror d, which reflects them into the eye under observation. The observer B, looks through the aperture in the concave mirror, and moves the two lenses m and till they are in such a position that a distinct image of the retina appears. We are now in a position, with the aid of the ophthalmoscope, to make a thorough examination of the retina. Fig. 23 gives a tolerable representation of all that we are able to distinguish of the image. The background of the whole is of a dull red, whilst the point where the optic nerve enters is distinguished as a round, bright spot, and we may see rising out of its midst the retinal vessels, arteries a, and veins b, which extend over the entire retina. The yellow spot also, the point of most distinct vision, may be distinguished as a small bright spot. The ophthalmoscope has become an instrument of incalculable value to the oculist. Many changes in the retina and interior of the eye, which are due to disease, can be observed and examined by means of the ophthalmoscope; and, in fact, the medical treatment of the eye has made an immense advance since the discovery of this instrument. The eyes of many animals, those of cats, for instance, exhibit a peculiar brilliancy, which is particularly remarkable in the dusk. It was formerly thought that the eyes of such animals emitted light independently, as it was also thought that light could be emitted by the human eye, under the influence of passion. This brilliancy, however, in the eyes of these animals is caused by a carpet of glittering fibres, called the tapetum, which lies behind the retina, and is a powerful reflector. In perfect darkness no light is observed in their eyes, a fact which has been established by very careful experiments; but, nevertheless, a very small amount of light is sufficient to produce the luminous appearance in them. 89 CHAPTER IV. Formation of the Retina-The Blind Spot and the Yellow Spot- Position of the Perception of Vision in the Retina. THE retina is the nervous organ of the eye, a delicate and highly complicated apparatus, whose mechanism has hitherto been but little understood, whose formation has long been the object of laborious investigation, and is so still. Different layers can be distinguished in the retina. when a fine section of it is examined with the microscope. The innermost layer, which lies next to the surface of the vitreous humour, consists of nerve-fibres, in which the optic nerve loses itself. It penetrates all the membranes of the eye at a point a little to one side of the centre, where the blood-vessels also enter the eye, which can be seen with the ophthalmoscope. At the point of entrance of the optic nerve, therefore, the retina consists only of the fibres of the optic nerve, which from this point radiate outwards. One spot, however, which lies in the centre of the retina, is free from nerve-fibres, and surrounded by them in a circular form. On account of its colour it is called the yellow spot. This is the part of the retina with which we see most distinctly; we will therefore speak more fully of its structure. |