890 OF SENSATION, AND THE ORGANS OF THE SENSES. 



is to develop a projecting, and that of the other a receding surface. For it will 

 be observed that the relative size of the parts which appear to project is reduced, 

 whilst that of the apparently receding parts is augmented ; as is particularly the 

 case with the square truncated end of the pyramid, which is estimated by most 

 persons as from one-third to one-half larger in each of its dimensions in the 

 receding, than it is in the projecting pyramid, notwithstanding that the actual 

 sizes of the squares in the two sets of figures are precisely the same. For, sup- 

 posing H I to represent the real side of one of the small squares, which becomes 

 the truncated end of the pyramid ; when this is brought forward by the mind 

 into the position K L, as the truncated top of a projecting pyramid, being seen 

 under the visual angle H A I, its apparent size is reduced to F G ; whilst, on the 

 other hand, the very same square, carried back by the mind to the distance I) E, 

 as when it forms the truncated end of the receding pyramid, is mentally enlarged 

 to the dimensions B c, the visual angle B A c being the same as H A i. It is 

 obvious, from what has been stated in regard to distance, that our power of form- 

 ing a true estimate of the relative sizes is far greater with regard to near objects, 

 whose relative distances we can estimate with tolerable accuracy, than it is with 

 respect to more remote bodies, whose relative distances we have no means of 

 appreciating ; thus, the sun and the moon are of nearly the same apparent size 

 to us, though one is about four million times the distance of the other ; and we 

 may cover either disk with a sixpence held near the eye, so as to be seen under 

 the same visual angle, without the least power of estimating the relative sizes 

 of these objects, save by a calculation based on a knowledge of their relative 

 distances, because, whilst the one is near, the other is virtually almost infinitely 

 remote. The want of innate power in Man to form a true conception of either 

 size or distance, is well shown by the effect produced on the mind unprepared 

 for such delusions, by a skilfully painted picture ; the view of which is so con- 

 trived, that its distance from the eye cannot be estimated in the ordinary man- 

 ner; for the objects it represents are invested by the mind with their real sizes 

 and respective distances, as if their real images were formed upon the retina. 1 



891. From all these considerations, we are led to perceive the truth of the 

 quaint observation made by Dr. Brown that "vision is, in fact, the art of 

 seeing things which are invisible;" that is, of acquiring information, by means 

 of the eye, which is neither contained in the sensations of sight themselves, 

 nor logically deducible from the intimations which those sensations really con- 

 vey. We cannot too constantly bear in mind, in treating of this subject, that 

 we do not take cognizance by our optic nerves, as we do by the nerves of touch, 

 of material bodies themselves, but of the pictures or images formed by those 

 objects ; and whatever be the notions suggested by the picture, that can never 

 be transformed into anything else. These notions appear to be, in the lower 

 Animals, entirely of an intuitional or instinctive character ; in Man they are so 

 in a much less degree ; and although it is impossible to come to a precise con- 

 clusion on the subject, from the want of sufficient data, it is indubitable that a 

 large part of the knowledge of the external world, which he derives in the adult 

 condition from the use of his eyes alone, is really dependent upon the early 

 education of his perceptive powers, in which process, the sensations conveyed by 

 different organs are brought into relation with one another. 



892. The persistence, during a certain interval, of impressions made upon 

 the retina, gives rise to a number of curious visual phenomena. The prolonga- 

 tion of the impression will be governed, in part, by its previous duration. Thus, 

 when we rapidly move an ignited point through a circle, the impression itself 



1 This delusion has been extremely complete in some of those who have seen the pano- 

 ramic view of London in the Coliseum. A lively and interesting account of it is given in 

 the "Journal of the Parsee Shipbuilders," who visited England some time ago. 



