be confounded m itli a feeling in tlie other ? It most as- 

 suredly can, for not only Bonders and Adamiik, by their 

 vivisections, butHeriug by his exquisite optical experiments, 

 have proved that the apparatus of innervation for both 

 eyes is single, and that they function as one organ — a 

 doubla eye, according to Hering, or what Helmholtz calls 

 a Cydopenauge. The retinal feelings of this double organ, 

 singly innervated, are naturally undistinguished as respects 

 our knowing whether they belong to the left retina or to 

 the right. We use them only to tell us where their ol)jects 

 lie. It takes long practice directed specially ad hoc to . 

 teach us on which retina the sensations severally fall. Simi- 

 larly the different sensations which arise from the jJosi- 

 tions of the eyeballs are used exclusively as signs of the 

 position of objects ; an object directly fixated being local- 

 ized habitually at the intersection of the two oj^tical axes, 

 but without any separate consciousness on our part that 

 the position of one axis is different from another. All we 

 are aware of is a consolidated feeling of a certain ' strain ' 

 in the eyeballs, accompanied b}-- the perception that just 

 so far in front and so far to the right or to the left there is 

 an object which we see. So that a ' muscular ' jjrocess in 

 one eye is as likely to combine with a retinal process in the 

 other eye to effect a percejjtive judgment, as two processes 

 in one eye are likely so to combine. 



Another piece of circumstantial evidence for the feelings 

 of iimervatiou is that adduced by Professor Mach, as fol- 

 lows : 



"If we stand on a bridge, and look at the water flowing beneath, 

 ■we usually feel ourselves at rest, whilst the water seems in motion. 

 Prolonged looking at the water, however, commonly has for its result 

 to make the bridge with the observer and surroundings suddenly seem 

 to move in the direction opposed to that of the water, whilst the water 

 itself assumes the appearance of standing still. The relative motion of 

 the objects is in both cases the same, and there must therefore be some 



lies. The open eye remaining fixed, and the closed eye moving towards 

 the right or left, the object seen by tlie open eye appears also to move to- 

 wards the right or left " iPhysiol. Oi>tik, pp. 607-8.) 



