IN OPTICS. 106 



is here to be desired. Should the interval of 

 the centres of those lenses be greater, objects 

 at very considerable distances will be seen 

 double. 



There are two other observations relative to 

 glasses for the sight, which I wish to add to 

 what I have already said upon this subject. 

 Thejfirst is, that the single convex glasses with 

 which some persons read, must be very inju- 

 rious, if they be sufficiently large, to admit the 

 same object to be seen with both eyes. For as 

 both axes will then pass through them, one on 

 each side of the centre, the interval of the pu- 

 pils will be widened, and the refracting power 

 of the eyes be diminished ; so that here a dis- 

 advantage is to be added to the prejudice of the 

 convexity of the glass, not a benefit to be placed 

 against it, as in the case of common spectacles 

 for the long-sighted. If, indeed, the defect in 

 sight does not arise from the conformation of 

 the eye, but from a want of transparency in its 

 cornea or humours, then such glasses, by mag- 

 nifying objects, will be useful, for the same 

 reason, that, in a very faint light, we can read 

 a book of a large print, with more ease than 

 one of a smaller. The second observation is, 

 that if flat-sided prisms were fixed in spectacle- 

 frames, with their refracting angles toward each 

 other, they would assist the long-sighted some- 



