ADJUSTMENT OF EYE TO DISTANCE. 443 



applied, or taken internally, dilates the pupil ; but it also destroys the 

 power of accommodation of the eye to distances, distinct near vision 

 being rendered impossible. The extract of the Calabar bean, on the 

 other hand, contracts the pupil, and, at the same time, the power of 

 distinct distant vision is diminished, whilst that of near vision is in- 

 creased ; the power of accommodation is not, however, entirely para- 

 lyzed. The visible effects of these substances are upon the iris ; but 

 it does not follow that they are limited to the muscular fibres of that 

 structure, so that the changes in the accommodating power of the eye 

 produced by them, must not be entirely attributed to the dilatation or 

 contraction of the pupil. There probably occur simultaneous effects 

 on the ciliary muscle, the conjoint results being, not only a change in 

 the size of the pupil, but also in the form of the crystalline lens. 

 Opium is another medicine which contracts the pupil, but its effects on 

 vision do not appear to have been accurately studied. Hydrocyanic 

 acid dilates the pupil widely, the circular rim almost disappearing. 



It is not yet understood how these medicinal agents affect the pupil. 

 Atropine, for example, may cause dilatation, either by paralyzing the 

 oculo-motor nerve, or its nervous centre, which might be termed passive 

 dilatation, or by stimulating the sympathetic nerve or nervous centre, 

 constituting an active form of dilatation, or in both ways simultane- 

 ously. But atropine is found to influence the state of the pupil, after 

 division of both the above-named nerves, and even after excision of 

 the eyeball. It is possible that these agents affect the pupil only in- 

 directly, by their blunting, or exalting, the sensibility of the retina, 

 and so causing, in the former case, dilatation, in the latter, contrac- 

 tion, of the pupil. 



The accommodation of the eye in viewing near objects, is known as 

 positive accommodation, that in looking at distant objects, as negative 

 accommodation. The act of accommodation is effected more rapidly, when 

 the eye is turned from a near to a distant object, than when it is directed 

 from a distant to a near one. The time required is probably modified by 

 age, practice, and other circumstances ; in old age, for example, more 

 time is occupied in accommodating the eye for near objects, than in child- 

 hood. The accommodation, when the eye is turned from an object at 60 

 feet distance, to one at 4 J inches, is said to take place in T 9 oths of a second; 

 whereas only T 7 5 ths of a second elapse in changing the accommodation 

 from 4J inches to 60 feet (Vierordt). It would seem, however, that, 

 in the case of short distances, the time necessary is relatively much 

 greater ; for according to other observations, the accommodation from 

 17 inches to 4J inches, requires as long as 2 seconds : that from 4J to 

 17 inches, Ijth of a second (Aby). Between a distance of 200 feet 

 and the horizon, all objects are seen equally clearly without change of 

 accommodation ; but at nearer distances, the necessity for exact ac- 

 commodation increases, and, in very near vjsion, it is absolutely essen- 

 tial, one object only in the so-called line ef accommodation (Czermak), 

 being clearly visible at the same time. 



The range of healthy vision, or the sight of hemitropic eyes (Don- 

 ders), is limited by two points, named the far point, or point of rest, 

 and the near point ; the ordinary limits of near vision, or close focal 



