284 RECENT PHOGRESS OP THE THEORY OP VISION. 



the most important cause of our perception of a third 

 dimension in the field of vision, has been furnished by 

 Wheatstone's invention of the stereoscope.^ I may assume 

 that this instrument and the peculiar illusion which it 

 produces are well known. By its help we see the solid 

 shape of the objects represented on the stereoscopic 

 slide, with the same complete evidence of the senses with 

 which we should look at the real objects themselves. 

 This illusion is produced by presenting somewhat dif- 

 ferent pictures to the two eyes — to the right, one which 

 represents the object in perspective as it would appear 

 to that eye, and to the left one as it would appear to the 

 left. If the pictures are otherwise exact and drawn from 

 two different points of view corresponding to the position 

 of the two eyes, as can be easily done by photography, we 

 receive on looking into the stereoscope precisely the same 

 impression in black and white as the object itself would 

 give. 



Anyone who has sufficient control over the movements 

 of his eyes does not need the help of an instrument in 

 order to combine the two pictiu:es on a stereoscopic slide 

 into a single solid image. It is only necessary so to 

 direct the eyes, that each of them shall at the same time 

 see corresponding points in the two pictures ; but it is 

 easier to do so by help of an instrument which will 

 apparently bring the two pictures to the same place. 



In Wheatstone's original stereoscope, represented in 

 Fig. 35, the observer looked with the right eye into the 

 mirror 6, and wdth the left into the mirror a. Both 

 mirrors were placed at an angle to the observer's line of 

 sight, and the two pictures were so placed at k and g 

 that their reflected images appeared at the same place 

 behind the two mirrors ; but the right eye saw the 



* Described in the Philosophical Transactions for 1838. — Tb. 



