266 Dr. Reade's Experiments for [April, 



the large blood-vessels and nerve running over its surface, from its 

 being of a grey colour, like pounded glass or animal jelly, would be 

 very unfit, and could never form an image of a grey object per- 

 fectly similar to itself, neither could objects the colour of the 

 choroid coat ever be seen. We might as well think of writing 

 with black ink on a sheet of black paper, as attempt the forma- 

 tion of dark images on a dark ground. On the other hand, how 

 admirably fitted both for reflection and transmission is the cornea, 

 both sufficiently transparent and sufficiently opaque; no coloured 

 substance could answer the purpose. It has hitherto been the 

 received opinion, that the two optic axes, concurring at the 

 object, make an angle, according to the size of which the object 

 appears large or small ; but this opinion, whose inconsistency 

 has been already pointed out by Bishop Berkeley, must yield to 

 the more rational theory, that the mind takes the apparent mag- 

 nitude and distance from the size of the corneal image, and not 

 from lines and angles beyond the nervous imiuence, or from 

 invisible rays, all rays being invisible, which are transparent until 

 intercepted and reflected. " In vain (says Berkeley) shall all 

 the mathematicians in the world tell me that I perceive certain 

 lines and angles which introduce into my mind the various ideas 

 of distance, so long as I myself am conscious of no such thing." 

 Indeed we might as well believe in ghosts and hobgoblins as 

 beheve that we could see an object, or the image of an object 

 beyond the nerves ; that is, beyond the transparent cornea. Here 

 is the rubicon, the utmost limit beyond which the mind can 

 never travel. Surrounding objects are brought to the eye by 

 means of the rays of light : hence the nerves convey them to the 

 sensorium. Indeed the idea that the mind could travel beyond 

 the cornea, ride on the whirlwind, and, like a fairy mab, measure 

 invisible angles of an invisible and distant image, is so very 

 inconsistent that we cannot but express surprise at its adoption. 

 If a man were gravely to say that he could touch the moon, he 

 would be looked on as mad ; but an astronomer says, that on 

 looking through a telescope he can measure the invisible image 

 of that body nearer to the eye than the moon, and beyond the 

 influence of the nerves ; and the astronomer gets credit for the 

 assertion. As the knowledge of distance almost entirely arises 

 from experience, founded on the analogy between the sense of 

 sight and touch, the former at a very early period of existence is 

 inadequate to regulate our perceptions. When an infant begins 

 to notice, natural education commences, external objects are the 

 letters, and the nerves the instructors of the mind. The insufficiency 

 of sight is evident by the anxious desire to feel and to examine 

 every new plaything. The image of the rattle is delineated on 

 the cornea, and the child believing it to touch the eye, grasps at 

 it although far removed. On the same principle I have heard a 

 child cry bitterly for the moon to play with. In a few months, 

 the sense of touch has partly educated the eye in judging dis- 



