VISION. 



1443 



contraction of the pupil, arresting the progress 

 of those rays which are too divergent, and 

 limiting those entering the eye to the central 

 rays, which, from their trifling divergence, 

 unite correctly on the retina. The same ad- 

 vantage is gained hy a near-sighted, and by a 

 presbyopic eye. In the one case vision is im- 

 proved by the card stopping the rays, which 

 would converge to foci in front of the retina ; 

 in the other, by its arresting those which tend 

 to foci behind the retina. So that this simple 

 experiment frequently makes a difference of 

 several inches in the vision of myopic and 

 presbyopic persons. 



A curious experiment devised by Father 

 Scheiner, has reference to this point. If we 

 make in a card two small orifices with a 

 needle, at a less distance from each ether 

 than the diameter of the pupil, and hold 

 these openings close to the eye, a double 

 image of a small object held within the 

 visual distance, a pin's head, for instance 

 will be seen. From the pin's head there pass 

 two very minute pencils of rays through the 

 apertures into the eye. These rays converge 

 towards a point lying behind the retina, and 

 fall upon the retina at two different points. 

 These are two isolated points of the circle of 

 dispersion, which would exist upon the retina, 

 if the other rays were not intercepted by the 

 card. If we now withdraw the pin's head more 

 and more, the images will approach, because 

 the rays, falling upon the eve through the 

 apertures, will diverge less, and will conse- 

 quently be refracted towards a point lying 

 nearer to the retina. If the object be re- 

 moved from the eye to the distance of distinct 

 vision, the two images will perfectly coincide, 

 since all rays passing from one point, lying 

 exactly at the distance of distinct vision, will 

 be concentrated at one point of the retina. It 

 may be asked, what are the conditions of 

 adaptation necessary for an eye in looking 

 through a fine aperture? In its normal con- 

 dition, for the maintenance of which no effort 

 is necessary, the eye is in the state necessary 

 for seeinu objects which lie at the distance of 

 distinct vision. If a distant object be re- 

 garded through the small apertures, the rays 

 passing through them into the eye must evi- 

 dently meet at one point before the retina, as 

 the condition of each adaptation does not 

 change in the eye : but the two pencils di- 

 verge again behind the point of intersection, 

 striking the retina at two different points, 

 when, consequently, distant objects will be 

 seen double ; therefore, we only see a small 

 object single, through two small apertures, 

 when it lies at the distance of distinct vision. 



This experiment of Scheiner led Dr. Porter- 

 field to invent an instrument called an op- 

 iometcr, for the determination of the focal 

 distance of the eye ; and Dr. Young sub- 

 sequently greatly improved upon it, his in- 

 strument being simple in construction, and 

 both convenient and accurate in its applica- 

 tion. 



The greatest distance of human vision is 

 so variable, that no arbitrary limits can be as- 



signed to it. Uncivilised tribes, as the North 

 American Indians, the inhabitants of the im- 

 mense Asiatic steppes, and the New Zealan- 

 ders, possess powers of sight which arc 

 almost incredible. It is interesting, however, 

 to remark, that the mean degree of capability 

 of vision was the same among the ancient 

 Greeks and Romans as at the present day. 

 The Pleiades furnish the proof of this, showing 

 that some thousand years ago, as now, stars 

 which astronomers call of the seventh mag- 

 nitude, were not visible to the naked eye in 

 persons of ordinary [lowers of vision. Even 

 among civilised nations, however, instances 

 are occasional!}' met with of extraordinarily 

 keen sight. General Drinkwater, in his 

 ' History of the Siege of Gibraltar," mentions 

 that there were two boys in the garrison pos- 

 sessed of such uncommon quickness of sight, 

 that they could see the shots fired by the 

 enemy almost immediately after they had 

 quitted the guns, and were constantly em- 

 ployed to look out and give warning to the 

 soldiers of the approach of these missiles. 



From the experiments of Harris, it seems 

 that a simple object, as a black square on a 

 white ground, or a white square on a black 

 ground, can be seen under a less angle than 

 the equal parts of a compound object, as the 

 squares of a chequered figure, and that their 

 least, or minimum visible angle cannot be less 

 than 40'' : others, however, say 30". If it is 

 40", the size of the image on the retina will 

 be ^oW inch. At a medium, Harris thinks it 

 not less than 2'. He remarks that the diffi- 

 culty of keeping the eye perfectly steady, may 

 be one cause why a single object can be dis- 

 cerned under a less angle than the parts of a 

 complex one ; and that it is natural to sup- 

 pose that the fewer objects we contemplate, 

 and the more they differ in colour, the easier 

 we can distinguish their several impressions on 

 the retina. The result of repeated and very 

 careful experiments by Hueck, tends to show 

 that white objects on a black ground are seen 

 at a greater distance than black objects on a 

 white ground *, and this is fully corroborated 

 by an instance mentioned by Humboldt. This 

 distinguished traveller was with Bonpland, at 



* The following facts, deduced from extensive 

 and very careful experiments, conducted by Lieut. - 

 Colonel Hamilton Smith, are of great practical im- 

 portance. The object was to ascertain the liability 

 of different colours to be hit as marks, under pre- 

 cisely similar circumstances, as to men engaged, size 

 of target, weather, &c. The result showed the 

 proportion to be as follows . 



Red, 12. Rifle-green, 7. Austrian blucish -grey, 5. 



Colonel Derinzy, who was actively engaged in 

 the Peninsular War, has given much attention to 

 the effect of different coloured uniforms on the 

 chances of being hit. The day before the battle of 

 Vittoria, his Portuguese rifle company, dressed in 

 earthy-brown, and a company of British Fiisileers, 

 of equal strength, dressed in red, had to dislodge the 

 French from a bridge. They were equally exposed 

 during the whole of the skirmish, and after it was 

 over, Colonel Derinzy compared notes with his 

 Fusileer comrade, and found that the relative losses 

 were precisely one to two ! This fact (for which we 

 are indebted to Captain Nelson, R.E.) strikingly 



4 z > 



