SENSE OF VISION. 



245 



Fig. 45. 



adapted to compensate for the excess of that of the organ itself, between the 

 object and the eye. On the other hand, in the presbyopic eye, the curvature 

 and refractive power are not sufficient to bring to a focus on the retina, rays 

 which were previously divergent in a considerable or even in a moderate 

 degree; and indistinct vision in regard to all near objects is, therefore, a 

 necessary consequence, whilst distant objects are well seen. This defect is 

 remedied by the use of convex lenses, which make up for the deficiency of 

 the curvature. We commonly meet with myopia in young persons, and with 

 presbyopia in old ; but this is by no means the invariable rule ; for even aged 

 persons are sometimes short-sighted; and long-sightedness is occasionally met 

 with amongst the young. In choosing spectacles, for the purpose of correcting 

 the errors of the eye, it is of great consequence not to make an over-compen- 

 sation ; for this has a tendency to increase the defect, besides occasioning 

 great fatigue in the employment of the sight. It may be easily found, when 

 a glass of the right power has been selected, by inquiring of the individual 

 whether it alters the apparent size of the objects, or only renders them distinct. 

 If it alter the size (increasing it if it be a convex lens, and diminishing it if it 

 be a concave), its curvature is too great; whilst if it do not disperse the haze, 

 it is not sufficiently powerful. In general it is better to employ a glass which 

 somewhat under-compensates the eye, than one which is of a curvature at all 

 too high ; since, with the advance of years in elderly persons, a progressive 

 increase in power is required ; and, as young persons grow up to adult age, 

 they should endeavour to dispense with the aid of spectacles. 



332. Many other interesting inquiries, re- 

 specting the action of the eye as an optical 

 instrument, suggest themselves to the physical 

 philosopher ; but the foregoing are the chief in 

 which the physiologist is concerned , and we 

 shall now proceed, therefore, to consider the 

 share, which the retina and optic nerve per- 

 form in the phenomena of vision. The Optic 

 Nerve, at its entrance into the eye, divides 

 itself into numerous small fasciculi of ultimate 

 fibrils ; and these spread themselves out, and 

 inosculate with each other by an exchange of 

 fibrils, so as to form a net-like plexus, which 

 is the outer layer of the true retina. From 

 this plexus, in which the fibres are lying in the 

 a plane of the surface of the vitreous humor, 

 very large number of fibrils arise, in a direction 

 perpendicular to that surface, so as all to be 

 directed towards the centre of the eye. These 

 pass through a delicate layer of cellular tissue, 

 containing a minute plexus of blood-vessels ; 

 and from this every fibril receives a sheath, 

 which envelops its extremity, thus forming 

 a minute papilla. The surface of the retina 

 nearest the vitreous humor, is entirely com 

 posed of these papillae, which are closely set 

 together ; a layer of cells is interposed between 

 them, however ; and these cells are regarded Papilla? of the retina of the Frog, seen 

 by Valentin as analogous to those of ganglionic from the side ^ ur " ed t( > w * rds the vitre ' 



J , , ?. , j r f ous humor; the four hipher rows are 



matter; but other Microscopists dissent from geen sidew ' ayg Magni ^ ed 300 times 

 this opinion, considering them as belonging {After T reviranus.) 

 rather to the vitreous humor. In the retina of 



21* 



Part of the retina of a Frog, seen from 

 the outer surface. Magnified 300 times. 

 (After Treviranus.) 



Fig. 46. 



