SENSE OF VISION ESTIMATION OF DISTANCE. 767 



spective, light and shade, etc., are faithfully depicted, if we are placed at a 

 distance from it, and are prevented from perceiving that it is but a picture. 1 

 In this case, however, a slight movement of the head is sufficient to unde- 

 ceive us; since by this movement a great change would be occasioned in the 

 perspective view of the object, supposing it to possess an uneven surface; 

 whilst it scarcely affects the image formed by a picture. In the same man- 

 ner, a person who only possesses one eye, may obtain, by a slight motion of 

 his head, the same idea of the form of a body which another would acquire 

 by the simultaneous use of his two eyes. 2 



624. Our appreciation of the relative Distances of near objects seems to be 

 derived in like manner from the mental combination of the perceptions de- 

 rived from the dissimilar pictures upon the two retinse, assisted by the sensa- 

 tions derived from the muscles of the eyeballs, which are put in action to 

 bring the optic axes into the requisite convergence. How much our right 

 estimation of the relative distances of objects not too far removed from the 

 eye, depends upon the joint use of both eyes, is made evident by the fact, 

 that, if we close one eye, we find ourselves unable to execute with certainty 

 many actions (such as threading a needle, or snuffing a caudle) which re- 

 quire its guidance. In proportion as the object is approximated to the eyes, 

 slight differences of distance produce marked differences in the degree of 

 convergence, and these are readily appreciated so as to afford the means of 

 very nice discrimination ; whilst, on the other hand, in proportion as they 

 are removed further and further, do the optic axes approach parallelism, 

 and the power of appreciating differences of distance is lost. It is the usual 

 opinion that the muscular sensation which accompanies the inclination of 

 the optic axes, immediately suggests the notion of the distance of an object ; 

 and that our appreciation of its size depends upon a secondary interpretation 

 of the magnitude of its picture on the retina, on the basis of this notion. 

 But it would appear from the experiments of Prof. Wheatstone, that the 

 reverse is the case ; the sensation of convergence assisting in the first in- 

 stance to determine the size, and the appreciation of distance being a 

 secondary judgment based on this foundation ( 627). The power of esti- 

 mating distance from the foregoing data, however, is obviously, in Man, not 

 an intuitive but an acquired endowment ; for it is evident to any observer, 

 that infants, or older persons who have but recently acquired sight, form 

 very imperfect ideas respecting the distance of objects; their attempts to 

 grasp bodies which attract their attention being for a long time unsuccessful, 

 so that they only gradually learn to measure distances by the sight, through 

 the medium of the touch. And it seems to follow from this, that even the 

 notion of " projection," which we seem necessarily to form when looking at 

 a solid object within a moderate distance, or at a properly adjusted pair of 

 Stereoscopic pictures, is not derived from an original intuition, but is the re- 

 sult of the association of our visual with our tactile experience, very early in 

 life, so as to constitute a "secondary intuition" on which all our subsequent 

 appreciation of projection is based. 



frequently brines the real scene fiir more forcibly before the mental vision than 

 when it is looked at with both eyes; since, in the latter case, the mind cannot avoid 

 perceiving the flatness of its surface; whilst in the former, the perspective, and the 

 distribution of the lights and shadows, are free to suggest to the mind the relative 

 distances and projections of the several parts. 



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

 panoramic view of London in the Colosseum. A lively and interesting account of 

 it is given in the Journal of the Parsee Shipbuilders, who visited England some 

 years ago. 



2 For many interesting experiments with the stereoscope, see Mr. T. Towne in 

 Guy's Hospital Reports, for 1862 and 1863. 



