Dr. J. Reatle on a new Theory of Telescopes. 21 



the cornea. For on an analogy with the other senses, such 

 as feeling, hearing, tasting, &c, we find extreme stimuli act 

 so as for a time to injure the organ, and in a manner to pa- 

 ralyse sensibility. Thus a soldier returning from the field of 

 battle, and deafened by the roar of cannon, for a time be- 

 comes insensible to minor sounds. The fingers accustomed to 

 rough usage, are incapable of nice works ; and in like manner 

 the retina accustomed to the stimulus of light sent from very 

 close objects, by degrees adapts itself and becomes insensible 

 to those more remote, and consequently less powerful. On 

 similar principles, a person with the best sight may make him- 

 self short-sighted, by merely wearing concave glasses. I have 

 met with some simple young gentlemen at College, who pro- 

 duced the disease by this affectation, and became perfectly 

 short-sighted. It is well known that watch-makers are short- 

 sighted, and sailors the reverse. A long-sighted person may 

 become short-sighted in a week, or after a lever, or a nervous 

 disease, without any change of the cornea or crystalline lens. 

 Nor can I conceive that wearing a concave glass could after 

 any time change the shape of the cornea. 



For the purpose of particularly investigating this subject, 

 I requested a very short-sighted gentleman to seat himself 

 opposite the letter T pasted on the window, as already men- 

 tioned in my paper On Vision, published in the Annals of 

 Philosophy ; and on looking at the correct reflected image on 

 the pupil, and comparing it with that of a long-sighted per- 

 son seated at the same distance, I could not perceive the least 

 difference either in the size or strength of colouring; yet the 

 long-sighted gentleman saw the letter distinctly, while the 

 other said that he only saw a very confused and large outline. 

 Hence we must infer that the disease lies not in any plump- 

 ness or flatness of the cornea, but in either the humours or 

 the nerves, or both. A convex glass increases the confusion 

 with a short-sighted person, and therefore is never used ; with 

 a long-sighted person it renders the object more distinct. 

 By the interposition of a concavo-concave glass, an image is 

 formed very near the eye, which, although small, sends the 

 rays with as much force to the cornea, as if the object itself 

 were in the same place : short-sighted people choose a small 

 print, write a small hand, and take off' their spectacles when 

 reading. Much has been written to very little purpose on 

 the means of finding the foci of spectacles, and philosophers 

 have exerted all their ingenuity; yet the practical optician, 

 however qualified to read their learned essays, throws them 

 aside for the old-fashioned and purely mechanical method of 

 holding the lens before a wall, and measuring the focus with 



a rule : 



