1442 



VISION. 



violet, and the light before the shadow as red 

 and yellow. 



Distinct vision. Dngald Stewart, in his 

 "Philosophy of the Human Mind," proposes 

 this question : Suppose the eye to be fixed in 

 a particular position, and the picture of an 

 object to be painted on the retina, does the 

 mind perceive the complete figure of the ob- 

 ject at once, or is this perception the result of 

 the various perceptions we have of the dif- 

 ferent points in the outline ? He arrives at 

 the conclusion that the mind does at one and 

 the same time perceive every point in the 

 outline of the object, for perception, like con- 

 sciousness, is an involuntary operation : as no 

 two points, however, of the outline are in the 

 same direction, every point by itself constitutes 

 a distinct object of attention to the mind ; but 

 these acts of attention are performed with 

 such rapidity, that the effect is the same as if 

 the perception were instantaneous. When 

 the eye is directed to any point of a land- 

 scape, it sees with perfect distinctness only 

 that image of it which is directly in the axis 

 of the eye; but the extreme mobility of the 

 eye, together with the duration of the impres- 

 sions upon the retina, enable us 'to take in 

 every part of the view with equal distinctness. 

 In all probability, it is only in the axis of the 

 eye, corresponding with the yellow spot, that 

 vision is perfect; the posterior part of the 

 retina is certainly better adapted to receive 

 images than the anterior, where the grey 

 nervous layer becomes thinner and thinner 

 towards the border. Dr. Young calculated 

 that the range of motion in the eye-ball is 

 55 in every direction, so that the head being 

 fixed, a single eye may have perfect vision of 

 any point within a range of 110. He fur- 

 ther ascertained, by fixing the eye in the 

 most natural direction, namely, forwards, and 

 a little downwards, and by then moving a 

 luminous object before it in various direc- 

 tions, that the range of vision upwards was 

 >0, downwards 70, inwards 60, and out- 

 wards 90, giving an entire horizontal play of 

 150, and a total vertical play of 120. 



The small portion of retina corresponding 

 to the extremity of the optic nerve, is insen- 

 sible to visual impressions. Volkmann states 

 that he has satisfied himself by calculation 

 that the small insensible spot corresponds 

 exactly with the dimensions of the central 

 artery. Dr. Young determined that the dis- 

 tance of the centre of the optic nerve from 

 the visual axis is J^ of an inch, and that the 

 diameter of the most insensible part of the 

 retina is ^ of an inch. It is to be borne in 

 mind that the fibrous lamina of the grey 

 nervous layer of the retina is at this point 

 evolving itself from the nerve, and is not yet 

 invested with the vesicular or other laminaa, 

 Mr. Bowman has well remarked that this in- 

 capacity of vision at the entrance of the optic 

 nerve seems to be essential to the mode of 

 junction of the retina with the nerve, since it 

 appears to have been the chief reason why 

 the nerve was not made to enter in the axis 

 of the eye. If the blind spot had been situated 



in the axis, a blank space would have always 

 existed in the centre of the field of vision, 

 since the axis of the eyes in vision are made 

 to correspond. But as it is, the blind spots 

 do not correspond when the eyes are directed 

 to the same object ; and hence the blank 

 which one eye would present, is filled up by 

 the opposite eye. Mariotte, was the first who 

 described the existence of these blind spots, 

 and they may be discovered by the following 

 simple experiment : Let two black dots be 

 placed one inch apart on a sheet of white 

 paper : if the left eye be closed, and the dots 

 are regarded at the usual distance for distinct 

 vision, the attention, however, being particu- 

 larly directed to that on the right hand, the 

 other dot will be found to disappear the 

 moment the pencil of rays proceeding from it 

 falls upon the centre of the entrance of the 

 optic nerve. 



It has been already stated, that when the 

 rays from an object meet in foci on the retina, 

 a distinct image is formed : should the focus, 

 however, be before or beyond the retina, it is 

 evident that some indistinctness must be 

 caused, for each point of the retina would 

 then receive rays from several points of the 

 object. It is to be borne in mind, that the 

 nearer the object to the eye, the greater is 

 the divergency of the incident rays ; and the 

 greater their divergency, the more distant is 

 their focus. When the retina corresponds, 

 or nearly corresponds, to the points of con- 

 vergence of the several pencils of light, dis- 

 tinct vision is obtained, the usual distance 

 being from eight to ten inches, at which dis- 

 tance, reading or writing is naturally per- 

 formed. If, when writing, for instance, we 

 removed the head further, or approached it 

 nearer, and no alteration took place in the 

 eye, vision would become indistinct, because 

 the focus would be altered, and would either 

 fall short of, or be thrown beyond, the re- 

 tina : should the rays, however, very nearly 

 unite upon the retina, vision, of large ob- 

 jects especially, may prove sufficiently dis- 

 tinct, though not perfectly clear. A just 

 distinction was, therefore, drawn by Jurin 

 between perfect vision and distinct vision, the 

 perfection of vision depending on the distance 

 alone of objects, whilst their distinctness 

 would be regulated not less by their size, than 

 by their distance from the eye. 



The pupil of the eye performs an important 

 part in enabling us to see objects distinctly. 

 When looking intently at a near object, the 

 pupil contracts, thereby limiting the rays 

 which pass through it to the most central, 

 and stopping the progress of those more di- 

 vergent rays which, not converging to foci on 

 the retina, would cause circles of dissipation ; 

 upon the same principle, vision of near objects 

 is assisted by a pin-hole aperture. Let a per- 

 son thrust a pin through a card, and make 

 a clean hole ; then let him hold a book so 

 close to his eyes that the type becomes con- 

 fused ; if he now looks through the pin-hole, 

 he will again see it distinctly, because the 

 card answers the same purpose as a further 



