274 SENSE OF SIGHT. 



does not fall between the eyes, on the root of the nose, as it ought to 

 do if the body were looked at with both eyes ; but on each eye alter- 

 nately, according as the one or the other is directed to it; and, he 

 adds, if, when we squint voluntarily, we see two objects, it is because 

 one eye sees passively, whilst the other is in activity. 1 



Amongst numerous objections to this view of the subject, a few may 

 be sufficient. Every one must have observed how much more vividly 

 an object is seen with both eyes than with one only. The difference 

 according to Jurin 2 is a constant quantity; and, in sound eyes of the 

 ordinary degree of power, amounts to one-thirteenth of the whole effect. 

 But we have experiment to show, that a distinct impression is made 

 upon each eye. If a solar beam be admitted into a dark chamber, and 

 be made to pass through two glasses of tolerable thickness, but of dif- 

 ferent colours, placed close alongside each other, provided the sight be 

 good and the eyes of equal power the light perceived will not be of the 

 colour of either of the glasses, but of an intermediate shade; and if this 

 should not happen, it will be found, that the eyes are of unequal power. 

 When such is the case, the light will be of the colour of the glass placed 

 before the stronger eye. These results were obtained in the Cabinet 

 de Physique of the Facult de Medecine of Paris, by M. Magendie, 3 in 

 the presence of M. Tillaye the younger. 



The existence of this double impression is proved in another way. 

 If we place any tall, slender object a few feet before us, and examine 

 its relative situation compared with a spot on a wall in the distance, 

 we find, that if the spot be hidden by the stick, when both eyes are 

 open, it will become visible to each eye, when used singly; and be seen 

 on the side of the stick corresponding to the eye employed. But Pro- 

 fessor Wheatstone 4 has instituted experiments, which place this matter 

 entirely at rest. He has shown, that in viewing an object having 

 length, breadth, and thickness, the perspective projections upon the 

 two retinae differ according to the distance at which the object is placed 

 before the eyes. If so distant, that to view it the optic axes must be 

 parallel, the two projections are precisely similar; but if so near, that 

 to view it the optic axes must converge, a different perspective pro- 

 jection is presented to each eye, and these perspectives become more 

 dissimilar as the convergence of the optic axes becomes greater. Not- 

 withstanding this dissimilarity between the two pictures, which is in 

 some cases very great, the object is still seen single, although not 

 exactly resembling either of the two pictures on the retinae. 



Having thus established, that the mind perceives an object of three 

 dimensions by means of the two dissimilar pictures projected by it on 

 the two retinae, Mr. Wheatstone inquired what would be the visual 

 effect of presenting simultaneously to each eye instead of the object 

 itself its projection on a plane surface as it appears to that eye ? For 

 this purpose he imagined an instrument which he calls stereoscope. It 



1 Adelon, Physiologic, 2de edit., i. 457, Paris. 1829. 



2 Essay appended to Smith's Optics, Cambridge, 1738; and Haller, Element. Physio!., lib. 

 xvi. 4. 



3 Precis, &c., i. 86. Dutours, in Mem. presentees & TAcadem., iii. 514, & iv. 499. 

 * Philosophical Transactions, P. ii., Lond., 1838. 



