THE FEELING OF EFFORT. 13 



The translocation would, in a word, be perfectly explained, could we suppose that the 

 sensation of a certain degree of rotation in the left eyeball were able to suggest to the 

 patient the position of an object whose image falls on the right retina alone. Can then, 

 a feeling in one eye be confounded with a feeling in the other ? 



Not only Bonders and Adamiik, by their vivisections, but Hering 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 double eye, according to Hering, 

 or what Helmholtz calls, a Ciidopenauge. Now the retinal feelings of this double 

 organ, singly innervated, are also to a great extent absolutely indistinguishable, 

 namely, where they fall in corresponding points. But even where they are numerically 

 distinguishable, they are indistinguishable with respect to our knowing whether they 

 belong to tlie left retina or to the right. When, as so often happens, part of a 

 distant object is hidden from one eye by the edge of an intervening body, and seen 

 only by the other eye, we rarely know by our spontaneous feeling that this is the 

 case, nor when we have noticed the fact can we tell which eye is seeing and which 

 is eclipsed. If the reader will hold two needles in front of his nose, one of them 

 behind the other, and look at the distant one with both eyes, the near one will 

 appear to him double. But he will be quite unable by his mere feeling, to say 

 to which eye either of the double images belong. If he gives an opinion, he will 

 probably say the right image belongs to the right eye, the reverse being really the 

 case.^ In short, we use our retinal sensations indifferently, and only to tell us where 

 their objects lie. It takes long practice directed specially ad hoc, to teach us on 

 which retina the sensations respectively fall. 



Now the different sensations which arise from the positions of the eyeballs are also 

 used exclusively as signs of the position of objects ; an object directly fixated, being 

 localized habitually at the intersection of the two optical axes, but Avithout 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 by 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. This being the case, our 

 patient paretic of the right external rectus, might be expected to see objects, not 

 only transposed to the right, but also nearer because the intersection of his squinting axes 

 is nearer, and smaller because a retinal image of fixed size awakens the judgment of an 

 object small in proportion as it is judged near. Whether paretic patients of this kind are 

 subject to this additional illusion remains to be discovered by examinations which 

 ophthalmologists in large practice alone have the opportunity of making.^ It is 



^ See also W. B. Rogers, Silliraan's Journal, I860, for other The extraordinary vacillation of our judgments of size 



curious examples of this incapacity. and distance will be noticed by any one who hiis experi- 



^ In three recent cases examined for me by opthalmologi- mented with slightly concave, convex or prismntic glasses. 



cal friends this additional delusion seemed absent, and I also The most familiar example is that of looking at the moon 



found it absent in a case of paralysis of the external rectus through an opera glass. It looks larger, so its details are 



with translocation which, by Dr. Wadsworth's kindness, I more distinctly seen ; being so distinct it looks nearer, and 



lately examined at the hospital. The " absence" spoken of because it seems nearer it is also judged smaller. (Aubert's 



was in all these cases a vacillating and uncertain judgment secundare UrtheilsUiuschung') . Many experiments may be 



rather than a steadfastly positive judgment that distance devised by which the left eye may be made to converge by 



and size were unaltered. a prism whilst the right looks either at the same object or sees 



