H52 



VISION. 



covers, and is inserted into, the anterior one- 

 eighth of an inch of the choroid membrane, 

 which is in this part tougher and firmer than 

 elsewhere, and united in a very special manner 

 to the lens by the ciliary processes, through 

 the medium of a firm tough membrane, and 

 of a strong elastic fibrous membrane proceed- 

 ing from it to the margin of the lens, and yet 

 not quite to its margin, for an elegant arrange- 

 ment exists, the canal of Petit, by which 

 traction is made, not on the vitreous around 

 the lens, nor on the edge of the lens itself, so 

 much as on its anterior surface. I confess it 

 seems to me very difficult to doubt that this 

 complicated system of parts is intended to 

 advance the lens towards the cornea, so as to 

 bring forward, up to the retina, the focus of 

 a near object, which would otherwise fall be- 

 hind the nervous sheet. It is possible also, I 

 think, from the peculiar direction taken by the 

 ciliary muscle, that it may compress the front 

 of the vitreous, and thus help to throw for- 

 ward the lens. 1 ' * 



An ingenious theory has been suggested by 

 Sturm, and supported by Matteuci, founded 

 on the results of Chossat's measurement of 

 the eye of an ox, to the effect that in place 

 of comparing the optical apparatus of the eye 

 to a system of spherical lenses whose axes are 

 blended, we ought to consider the organ as 

 composed of several refracting media, sepa- 

 rated by surfaces which are neither exactly 

 spherical, nor even of revolution or symme- 

 trical about a common axis. Reasoning from 

 this, he argues that a peculiar refraction of 

 the rays of light takes place, whereby tie 

 retina is placed in what he terms a focal in- 

 terval, which focal interval will change its 

 position according as the external luminous 

 point recedes from, or approaches to, the eye 

 and that the retina will be always met by the 

 concentrated fasciculus around the axis in the 

 focal interval ; the surface of intersection of 

 this fasciculus and of the retina being very 

 slightly modified, in order that the impression 

 may not be sensibly altered, or the perception 

 rendered indistinct'. This theory is, however, 

 decidedly open to objection, and is rendered 

 unnecessary by that of Mr. Bowman. 



Magnifying lens. It has been already 

 stated'th'at the apparent magnitude of an ob- 

 ject depends upon that of the angle of vision 

 under which it is seen, and this increases in 

 proportion as the object is brought nearer to 

 the eye ; but the magnitude of the angle of 

 vision being limited, we are obliged to resort 

 to artificial means to enlarge it further than, 

 in its natural condition, is admitted of. The 

 pin-hole aperture affords some assistance, but 

 the convex lens more. The following is the 

 mode in which it acts : 



Let c D be a convex lens, and A B an object 

 lying within the focal length of the glass, then 

 all the rays passing from a point of the object 

 AB will diverge after their passage through 

 the lens, exactly as if they came from the 

 corresponding point of the image ab ; an eye 



* Lectures on the Parts concerned in the Opera- 

 tions on the Eye, p. 60. 



Fig. 889. 



behind the lens would be able to see the ob- 

 ject distinctly through the lens if the image 

 a b were at the distance of distinct vision. In 

 this case, however, the object being much 

 nearer the eye, it could not be seen without 

 the lens. The magnifying power of the lens 

 therefore depends essentially on the means it 

 gives us of bringing the object very near the 

 eye, and thus increasing the angle of vision. 



ABNORMAL VISION. 



In the consideration of abnormal vision, we 

 propose to divide the subject into 



1. Abnormal Vision resulting from defec- 

 tive action of the retina or sensorium ; as 

 Achromatopsy, Hyperchromatopsy, and 

 Anorthopia. 



2. Abnormal Vision arising from faulty con- 

 figuration of the eye, or from changes in 

 the refractive media ; as Myopia, Pres- 

 byopia, and Cylindrical Eye. 



Amaurosis, Chrupsia, and other morbid 

 conditions, do not fall within the scope of this 

 article. 



Achromatopsy (a not, xp<^" a colour, &// the 

 eye), or insensibility of the eye to colours, is 

 an affection which has been recognised nearly 

 two hundred years ; but, although cases have 

 been from time to time published in the Phi- 

 losophical Transactions and other scientific 

 works, our knowledge of the phenomena of 

 this singular condition is of recent date, and 

 is chiefly due to the labours of Wartmann, 

 Seebeck, Szokalski, Purkinje, Himly, &c. 



Various names have been proposed for this 

 imperfection of vision ; but the majority are 

 exceedingly unmanageable. By Sommer and 

 Szokalski the term chromato-pseudopsis has 

 been employed ; Goethe proposed to call it 

 akyano-blepsis, whilst Purkinje divided the 

 disorder into four varieties, achromatopsia, 

 chromato-dysopsix, akyano-blepsis, and anery- 

 thro-blcpsis ; others again have been satisfied 

 with the simple term chromato-metablepsis. 

 Jiingken employs indifferently the denomina- 

 tions of achromatopsy, chromatopseudopsy, and 

 chromatometablepsy. Many writers, however, 

 have adopted the term Daltonism, proposed by 

 Prevost, and supported by Wartmann ; and, 

 although objectionable as perpetuating the in- 

 firmity of an individual, it has the merit of 

 simplicity and easy inflection. The term 

 achromatopsy is, perhaps, that most usually 

 employed, although, strictly speaking, it is 

 only applicable to one class. Still, being ex- 

 tensively recognised, we shall adopt it to de- 

 signate this imperfection of vision ; occasion- 



