THE PERCEPTION OF SIGHT. 249 



present different pictures to the two eyes, while a painting 

 shows only the same. Hence follows a difference in the impres- 

 sion made upon the sight which, the utmost perfection in a re- 

 presentation on a flat surface cannot supply. 



The clearest proof that seeing with two eyes, and the diffe- 

 rence of the pictures presented by each, constitute the most im- 

 portant cause of our perception of a third dimension in the 

 field of vision, has been furnished by Wheatsone's invention of 

 the stereoscope. 1 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 different 

 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 pictui-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 instru- 

 ment 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 b, and 

 with 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 

 1 Described in the FhilosojMcal Transactions for 1838. TR. 



