480 Dr Alison on Single and Correct Vision, by means of 



those which follow impressions made on different points of the sur- 

 face of the body. 



I. Of the first of these facts, i. e. the single vision by means 

 of double images, it is well known that an explanation was pro- 

 posed by Newton, fully considered by Re id, and since support- 

 ed by Wollaston (often called the Theory of Wollaston, but 

 quite incorrectly), proceeding on the supposition of a semi-decus- 

 sation of the human optic nerves at their commissure ; whereby 

 the fibres, from the right half of the retina of each eye, go to the 

 right optic lobe* in the brain, and vice versa ; the consequence 

 of which may probably be, that the fibres which connect them- 

 selves with, or terminate at corresponding points of the retinae, 

 may originate at the same points in the optic lobes. If this be 

 so, impressions made on corresponding points of the retinae are in 

 fact impressions made on the same points in the optic lobes ; 

 and, as they are effectual in exciting sensations in the mind, 

 only inasmuch as they are made on the optic lobes, they must 

 necessarily co-operate in exciting the same sensations. 



Dr Re id fairly admits that, if the anatomical part of this 

 theory were ascertained to be correct, " it would lead us a step 

 forward in discovering the cause of the correspondence and sym- 

 pathy of the two retinae ;" — he ought rather to have said — the 

 cause of single vision by impressions made on corresponding 

 points of the two retinae. I think we may add, that it is the 

 only step which we can conceive to be taken, or can desire to take, 

 in that inquiry. And I will farther venture to maintain, that a 

 precisely similar step may be taken, even with more confidence, 

 as to the correct vision by means of inverted images. 



We must admit, that the anatomical evidence of the theory 



* The term optic lobe is here used as a short expression for that portion (per- 

 haps not yet absolutely determined) of the contents of the cranium, from which the 

 optic nerves originate, and on which their sensibility depends. 



