XXXVI.] PEECEPTION. 123 



sensation. But in the most important class of perceptions Percep- 

 — that is to say, in those of sight — elements from the two to^both"*^ 



senses of sight and touch are combined. The peculiarly touch and 

 . . . -, , -I . • f ^ u_T sight. 



intimate connexion between those two senses is due to the g^g^jg^i 

 fact, that they alone of all the senses give any cognition connexion 

 of space.^ Space has become more associated in our ^^q seij^jjey_ 

 habitual thoughts with sight than with touch ; but the 

 origin of our cognition of space in its three dimensions 

 must be due, not to sight, but to touch (including the Cognition 

 muscular sense as belonging to the latter) ; for the hand, ^^ l^^^^ 

 which is the principal organ of touch, is capable of dimensions 

 moving in all three dimensions, and thus of giving origin touch, 

 to the cognition of three dimensions ; while the eye, 

 whether at rest or in motion, has no way of cognising a 

 dimension. It is true that the eye acquires a power of 

 estimating distance, not only by the effects of perspective, 

 but by means of the varying optical adjustments which 

 are spontaneously made for varying distances ; but this 

 power is later acquired, and is far inferior in accuracy, as 

 well as totally different in kind, from the power of the eyes 

 to appreciate the space-relations of surface. It has been 

 shown by observations made on persons born bhnd who 

 afterwards received sight by the removal of the congenital 

 cataract, that they knew simple superficial forms, such as a 

 circle or a square, when presented to their sight, but had no 

 perception whatever of the distances of objects from the eye.^ 



1 See the chapter on the Physiology of the Senses, in Dr. M 'Cosh's 

 Examination of Mill's Philosophy. 



2 See Dr. M 'Cosh's chapter, already referred to. Berkeley said that a 

 person born blind, and acquiring sight suddenly, would not know a circle 

 from a square. In his time, no observations on the subject had been 

 made. It has been subsequently stated, that his purely theoretical views 

 have been confirmed by observations on persons cured of congenital 

 cataract ; but a very carefully observed case reported by Dr. Franz of Leipzig 

 (quoted by Dr. M'Cosh from the Philosophical Transactions of 1841) appears 

 conclusive as to the cognition of superficial extension. With respect to 

 the inability of the eye to perceive the distance of objects from itself, 

 until it has learned to do so by practice, Mr. Abbot, in his work " Sight 

 and Touch," argues that no observation of the kind is in the least degree 

 conclusive ; because in all such cases as yet recorded there has been but 

 one eye, so that the patient has been without those means of perceiving 

 distance which arc given by the simultaneous use of two eyes ; and in that 



