SIGHT. 



SIGHT. 



4.VI 



bj the fire from the button ; it then proceed* in Serpentine direction 

 till iu head arrives between the portion* of the fourth parallel already 

 funueil. This parallel U then completed, and under the protwti. >n ! 

 the fire from the troop* stationed in it, the counter and breaching 

 batteries before the bastion are formed. By the former the fire from 

 the gun* in th flank* of the collateral bastions i* partly ailenced, and 

 by the latter the breach in the faoea of the opposite bastion it effected. 

 The passage through the counterscarp and a trench acroa* the main 

 ditch are then executed, and an asuull may be made up the breach of 

 the battion, similar to those which had been made up breache* of the 

 ravelin* ; the defender* being repelled, a lodgment may be formed, 

 and unless the bastion U strongly retrenched, it may be expected that 

 the place will now be surrendered. It u estimated that the ansault of 

 the bastion may take place about the twenty-sixth night from the time 

 of opening the trenches; but a good retrenchment [RITBENCUMKST] 

 in a bastion may enable the defenders to hold out ten or twelve days 



A fm 



fmties* i* said to be countermined when subterranean galleries 

 are formed under the ramparts of the bastion* and ravelins; under the 

 corered-way, and under the ground at the foot of the glacis, with 

 galleries of communication from one of these to another. And as the 

 defender* can form chambers and place powder in or near any con- 

 venient part* of these galleries to destroy the works of the besiegers 

 above-ground, the besiegers find themselves under the necessity of 

 sinking shafts and forming galleries for the purpose of finding out and 

 destroying those of the defenders, or of blowing up any of their ad- 

 vanced work*. [Mian, MILITARY.]! 



A siege conducted according to the rules of art will be attended 

 with comparatively small loss to the besieger* or besieged, the troops 

 of both parties being but little exposed to each other's fire except at 

 the times when the assault* are made on the ravelin* or bastions. And 

 if circumstances, *uch as th prospect of the place being relieved, did 

 not compel the besiegers to expedite the surrender, the assault* by 

 main force might be avoided ; for after a breach hag been formed, and 

 the parapet* of the place have been in a great measure ruined by the 

 artillery of the beeiegen, a sapper might be sent across the ditch by 

 night with instructions to commence a trench under cover of one ex- 

 tremity of the broken wall; then, if he succeed in getting covrr t'..r 

 himself, others may follow, and gradually there may be formed on the 

 breach a lodgment sufficiently large to contain troops, whose fire would 

 protect the succeeding operations : it being understood that a firing 

 party in the batteries on the glacis force the defender* to retire as 

 often a* they endeavour to disturb the sappers while at work. 



The want of time and mean* to carry on the approaches as for as 

 the covered-ways wo* the cause of the great losses sustained in getting 

 possession of the fortresses garrisoned by the French in Spain during 

 the Peninsular war. The breaches in the walls of Badajo* (1812) were 

 made by guns in batteries at the distance of 500 yards ; and when the 

 assailing troops had descended into the ditch, being ignorant of the 

 positions of the breaches, and confused by the darkness of the night, 

 which was relieved only by the appalling and destructive blaze of live 

 shells and other combustibles thrown upon them from the parapets, 

 they took a wrong direction, or remained patiently to be slaughtered 

 till the order was given to retire. The effort would have entirely 

 failed, but for the success of Major-General Pictou in escalading the 

 walls of the castle, and of General Walker in escalading the bastion of 

 S. Vincente. 



SIGHT (vision, the faculty of seeing). The structure and uses of 

 the several component parts which enter into the formation of the 

 organ of vision have been already described iu the article EYE, in NAT. 

 HIST. Div. We have now to inquire by what means the images of 

 objects which are depicted on the retina become converted into idea* 

 of the objects themselves ; of their proximity and distance ; of their 

 solidity and size. In other words, is the interpretation of the sensa- 

 tions of the retina a vital property of the structure itself, or i* it in ]>art 

 derived from other sources ! The following case, which is recorded by 

 Cheselden, affords us important data on this head. A young man, who 

 was born blind, was suddenly restored to sight by the operation of 

 couching. *' When he first saw," observes Cheselden, " he was so far 

 from making any judgment about distances, that he thought all object* 

 whatever touched his eye (as he expressed it), as what he felt touched 

 hi* skin. He knew not one thing from another, however different in 

 shape and magnitude ; but upon being told what things were whose 

 form be before knew from feeling, he would carefully observe, that 

 he might know them again. Two months after being couched, hi* 

 attention seem* to have been drawn to the effects of pointing, which he 

 then first and at once comprehended ; but even then he was no less 

 surprised, expecting (the pictures would feel like the things they re- 

 presented, and was amazed when he found those part* which by their 

 light and shadow appeared round and uneven, felt only flat like the 

 rest, and asked which was the lying sense, feeling or seeing ? Being 

 shown a small miniature of hi* father, and told what it was, he ac- 

 knowledged a likeness, but was vastly surprised, asking how it could be 

 that a large face could be expressed in so little room, saying it should 

 have seemed as impossible to him as to put a bushel into a pint. At 

 first he could bear but very little light, and the things he saw he 

 thought extremely large, but upon seeing things larger, those first seen 

 be conceived leu, never being able to imagine any line* beyond the 



bound* he saw. The room he was in, he said he knew to be but 

 put of the house, yet he could not conceive that the whole house 

 could look bigger." From the details of this interesting case, it wuld 

 appear that the sense of sight (so far as we can judge of it when per- 

 formed with one eye, for only one had been operated on win n tin- 

 above phenomena were observed) originally gives us no information 

 respecting the solidity , the distance, or the real magnitude of object* ; 

 but that they all seem as if |inted on one surface. If this then is the 

 sum of the information which i* conveyed to u* by the retina ; if it is 

 limited to the mere perceptions of the images of objects, and conveys 

 to us ideas relative to superficial extent only, it is clear that our 

 estimation of the true position, the magnitude, and solid forms of 

 bodies must be due to some other sense than that of sight, or rather, 

 to a comparison of some other sense with it ; in short, to an act of our 

 comparing and reasoning faculties. 



We have seen by the details of the above quoted case, that there is 

 no essential resemblance between the idea* which ore derived from 

 vision and those communicated by touch ; and it is no doubt jurtly 

 owing to this circumstance that we obtain a correct knowledge of 

 external objects through our visual organs. The lad couched by 

 Cheselden could not recognise by sight the things whose form he 

 before knew from feeling ; but upon being told what they were, he 

 would carefully observe, that he might know them again. The infant, 

 in like manner, stretches out its little hands to grasp and examine each 

 object in succession which attract* it* sight, and the greater port of 

 it* waking hours is employed in thus comparing the sensations obtained 

 through these two different channels. That we do acquire important 

 information respecting the size and forms of bodies through the sense 

 of touch there can be no doubt ; that the knowledge obtained through 

 our visual organs would be imperfect without it, and that it may in 

 some measure be a corrective of those optical illusions which are so 

 frequent when we attempt to judge of things by sight alone, is equally 

 probable ; but in admitting this, we must not underrate the original 

 powers of the eye and the quantity of knowledge which we primarily 

 derive from it. From some facts we are about to notice, it would 

 appear that much of the information which we derive from our visual 

 organs only, has hitherto been attributed to extraneous sources. Pliy- 

 siologists have been too much swayed by the opinion of Gasnendus, 

 Haller, Gall, and others, that we see with only one eye at a tin 

 those even who disputed this have been more anxious to explain why 

 objects are seen single with both eyes than to inquire into the uses of 

 our possessing two. These defect*, which are more or less common 

 to all writers on optics, have been some years since remedied by some 

 very interesting observations of Professor Wheatstone on Binocular 

 Vision. He has shown that the simultaneous affection of the two 

 retiiw, provided the optic axes are not parallel, excites a different, idea 

 iu the mind from that consequent on either of the single impressions ; 

 the latter giving rise to a representation on a plane surface, the former 

 to that of an object in relief. This is owing to a different per.-) 

 projection of the object being seen by each eye ; thus, if any figure of 

 three dimensions, on outline cube for example, is held at the distance 

 of about seven inches before the eyes, and viewed with each eye suc- 

 cessively while the head is kept perfectly steady, A will be the picture 

 presented to the right eye, and B that seen by the left. Now if 



B A 



these two pictures ore made to fall on corresponding part* of the 

 retinae, by placing them one in the direction of each optic axis at 

 oi i nil distances before or behind their intersection, the mind will 

 perceive not merely a single representation of the object, but a body 

 ing in relief, the exact counterpart of that from which the 

 drawings were made. 



But a better method is to employ the stereoscope, on instrument 

 invented for the purpose by Mr. Wheatetone, the essential parts of 

 which are two plain mirrors inclined with their back* towards each 

 other at an angle of 90*. The two pictures A and B are placed in the 

 same horizontal line, and parallel to each other at the sides of these 

 mirrors, and at equal distance* from them. The observer then placing 

 hi* eye* as near as possible to the two mirrors, their angle coinciding 

 with the middle line of his forehead and face, sees the solid body 

 represented by the perspective drawings standing forward in relief, 

 provided the two drawings are so situated that their images reflected 

 by the mirrors coincide with the line* of the convergent optic axes. 

 Instead of the original reflecting stereoscope of Mr. Wheatatone, an 

 instrument of the refracting form introduced by Sir David Brewster, 

 ;iinl now BO common, may be used in the experiment. When similar 

 images, differing to a certain extent in magnitude, are presented, by 

 means of the stereoscope, to corresponding part* of the two rctiiur, a 

 single object, intermediate in size between the two monocular picture*, 

 is seen. Wore it not for this, object* would be seen single only when 



