IN OPTICS. 99 



concave glasses to benefit the myopic, by 

 producing a contrary effect upon rays which 

 diverge from a distant object. Now it is ma- 

 nifest, that by this theory, to which I believe 

 no addition has been made by any succeeding 

 writer, precisely the same effects are attributed 

 to lenses, whether they be employed singly, or 

 in the form of spectacles. I am inclined, how- 

 ever, to think, that a difference, sometimes at 

 least, exists here, which has hitherto escaped 

 notice. For in regard to such spectacles as I 

 have tried upon myself, I have always found, 

 that, when I looked with them at objects placed 

 at moderate distances directly before me, my 

 optic axes passed through the glasses, more in- 

 wardly than their centres. With respect, there- 

 fore, to spectacles for long-sighted people, as 

 the inner halves of their glasses may be re- 

 garded as two prisms, whose refracting angles 

 face each other, to have allowed both my eyes 

 to receive through them pencils of rays from 

 the same point of an object, the intervals of my 

 pupils must have been less than was necessary 

 for that purpose in naked vision. The conse- 

 quence of which would be, an increase of the 

 refractive power of my eyes. Again ; as the 

 like parts of glasses in spectacles for short- 

 sighted persons, may be esteemed to be two 

 prisms, the refracting angles of which are 



