APPRECIATION OF DISTANCES. 283 



that it may depend upon a want of sensibility in the retinae, similar to 

 that observed in the ears of those who are incapable of hearing notes 

 above a certain pitch; but as this view is not contained in his more 

 recent " Treatise on Optics," it is probably no longer considered by him 

 to be satisfactory. 



The defect in question dffficult as it is to comprehend has always 

 appeared to the author to be entirely cerebral, and to strikingly re- 

 semble, as Dr. Brown has suggested, the " want of musical ear." As 

 we have already endeavoured to establish, that the latter is dependent 

 upon a defective mental appreciation, the parity of the two cases will, 

 of course, compel us to refer the visual defect, or the want of the " fa- 

 culty of colouring," to the same cause. It has been remarked, that 

 the eye in these cases exercises its function perfectly as regards 

 the form and position of objects, and the degree of illumination of 

 their different portions. The only defect is in the conception of colour. 

 The nerve of sight is probably accurately impressed, and the deficiency 

 is in the part of the brain whither the impression is conveyed, and 

 where perception is effected, which is incapable of accurately appreci- 

 ating those differences between rays, on which their colour rests ; and 

 this is the view taken of it by one of the most eminent philosophers 

 of the present day, Sir J. F. W. Herschel. 1 



The mediate or auxiliary functions of vision are numerous; hence, 

 the elevated rank that has been assigned to it. By it, we are 

 capable of judging, to a certain extent, of the direction, position, 

 magnitude, distance, surface, and motion of bodies. Metaphysicians 

 have differed greatly in their views on this subject ; the majority be- 

 lieving, that, without the sense of touch, the eye is incapable of form- 

 ing any accurate judgment on these points; others, that the sense of 

 touch is no farther necessary than as an auxiliary; and that a correct 

 appreciation could be formed by sight alone. The few remarks that 

 may be necessary on this subject will be deferred until the physical 

 and other circumstances which enable us to judge of distance, &c., have 

 been canvassed. 



The direction or position of objects has already been considered, so 

 far as regards the inverted image formed by them on the retina. The 

 errors that arise on this point are by no means numerous, and seldom 

 give rise to much inconvenience; yet, whenever the luminous cone meets 

 with reflection or refraction before reaching the eye, the retina conveys 

 erroneous information to the sensorium, and we experience an optical 

 illusion. 



To ascertain the magnitude, distance, and surface of bodies, we are 

 obliged to take into consideration several circumstances connected with 

 the appearance of the object, such as its apparent size; the intensity 

 of light, shade, and colour ; the convergence of the axes of the eyes ; 

 the size or position of intervening objects, &c. Porterfield 2 enumerates 

 six methods, which are employed in appreciating distance 1. The 



1 Encyclopaedia Metropolitana, art. Light. 



3 A Treatise on the Eye, ii. 409, London, 1759. 



