SENSE OF VISION. 753 



images will be seen in the central part of the field of vision ; whilst if the 

 axis of the eye be too short, as in Hypermetropia, the rays will not have 

 united, and double images will also be seen. By placing a piece of red or 

 green glass over one of the holes, as at </, it is easy to determine whether the 

 ray passing through that hole has or has not crossed. 1 The term " pres- 

 byopia," as limited by Bonders, simply expresses a deficient power of ac- 

 commodation in the eye, resulting from increased density of the lens, or 

 from defective power of the ciliary muscle, so that the "near-point" recedes 

 beyond a certain point, arbitrarily fixed by Bonders for the sake of conve- 

 nience at eight inches. The effects of atropin upon the eye are very re- 

 markable, not only in dilating the pupil to the utmost in the course of from 

 twenty to twenty-five minutes, but in completely paralyzing the power of 

 accommodation, so that the "near-point" becomes gradually more and more 

 distant, till at length it coincides with the " far-point." The Calabar bean, 

 on the contrary, causes extreme contraction of the pupil in the course of 

 from thirty to forty minutes, the " far-point," and in many eyes the " near- 

 point" also, becoming approximated to the eye, though the power of accom- 

 modation is never altogether lost. Both of these agents appear to exert a 

 stimulating as well as a paralyzing influence on the nerves supplied to the 

 Iris ; atropiu paralyzing the third and exciting the sympathetic nerve, whilst 

 the Calabar bean paralyzes the sympathetic and excites the third. 2 The 

 term astigmatism 3 (a, privative, ffrt^a, a point) has been applied by Pro- 

 fessor Whewell to a condition of the eye (first observed in himself by Pro- 

 fessor Airy) in which there is an inequality in the refractive power (owing 

 to difference in the degree of curvature either of the cornea, or of the lens, 

 or of both) between the horizontal and vertical meridians of the eye. There 

 is a consequent incapacity on the part of the eye to collect all the rays of 

 light entering it to one exact focus ; this has been shown by Bonders to be 

 of common occurrence in those who are otherwise healthy. The asymmetry 

 is usually of such a nature that with each degree of accommodation hori- 

 zontal lines are seen distinctly at a point nearer to the eye than vertical lines, 

 showing that the vertical meridian has a shorter focal distance than the 

 horizontal. This condition may be remedied by the use of the so-called 

 cylindrical glasses. Many other interesting inquiries, respecting the action 

 of the eye as an optical instrument, suggest themselves to the Physical phi- 

 losopher ; but the foregoing are the chief in which the Physiologist is con- 

 cerned ; and we shall now proceed, therefore, to consider the share which 

 the nervous apparatus performs in the phenomena of vision. 



615. The Optic Nerve, 4 at its entrance into the eye, divides itself into nu- 

 merous small fasciculi of ultimate fibrils ; and these appear to spread them- 

 selves out, and to inosculate with each other by an exchange of fibrils, so as 

 to form a netlike plexus, which constitutes the inner layer of the Retina (Fig. 

 266, 7) in immediate contact with the "limitary membrane" (8). There is 

 considerable difficulty, however, in the precise determination of the course 



1 See Thompson, Amer. Jour. Mod. Sci., April, 1870. 



8 See the Essays of Rogow, in Henle and Pt'euffer's Zeitschrift, Bd. xxix, 1867, p. 

 1 ; Bernstein in idem, p. 35 ; Dr. Argyll Robertson on Calabar Bean, Trans, of Royal 

 Soc. of Edin., vol. xxiv ; V. Grafe, Arehiv f. Ophth., Bd. ix, Heft iii, p. 87. 



3 See Donders's Astigmatismus, etc. (Berlin, 1862) ; the Oration delivered by Mr. 

 Z Laurence before the North London Medical Society, 1863, and his papers in the 

 Med. Times and Gazette for 1862-63 ; also Mr. Wharton Jones's abstract of paper 

 read before the Royal Society, in Proceedings of the Royal Society for 1859, vol. x, 

 p. 374. 



4 An excellent account of the historical development of our knowledge of the 

 structure of the retina, from the year 1856 to the year 1868, is given by Krauss, in 

 Robin's Journal, vol. vi, 1869, p. 438. 



