C 24:4t ON SPECTACLES. 



It has been demonftrated by mathematicians, that an oblique 



pencil of rays has its focus a little nearer the lens, than a 



direct pencil; but how much this difference amounts to, at 



different angles, feems not to have altracled their attention. 



This difference, however, is too considerable to be difregard- 



ed, when the rays of light fall very obliquely. 



Confiderations For let L L (Fig. 3. PL xi.) reprefent a double convex 



relating to j ens p its f ocus a p j ts ax j s anc } a b a ray parallel to it, 



oblique pencils. ,.,.,, J 



which wdl be refracted to F, after paffing through the lens. 



Alfo let c b and d d be two oblique rays falling upon the lens, 

 at b and d. 



Then the rays near A F falling almoft perpendicularly upon 

 the fides of the lens, are Iefs bent than (he raysc b, dd, which 

 fall obliquely on both fides of the lens. *' For the more 

 oblique they are, the more they are bent, and turned out of 

 their direct way. And consequently the oblique rays c b, dd, 

 ■will fooner interfect in G, than thofe at F." (Emerfon's 

 Optics, page 124.) When the angle a b c is very confider- 

 able, the difference between .r F, and y G, will likewife 

 be very confiderable. And the following fhort account of my 

 experiments will throw fome light upon the truth of this pro- 

 pofilion. 

 Spectacles In the fpeclacles that I ufe, the reading diffance of oblique 



obliquely placed. ravs j s f our j nc hes nearer the glafles, than the diftance -at 

 which I can fee belt, with the direct rays. 



And by experiments made with a lens of 24 inches focal 

 diftance, and 4 inches in diameter, contracted to % of an inch, 

 I found the focal diftance of an oblique pencil of rays 8 inches 

 fhorter than the focal diftance of the direct rays. The axis 

 of the lens, when placed for the oblique rays, made an angle 

 of 26° with a line drawn from its centre, to the candle with 

 which the experiments were made. And the diftance be- 

 tween the candle and the lens meafured about 10 feet, when 

 the lens was placed to receive the direct rays. 



Cor. 1. When an object is feen through fpeclacles, by 

 oblique rays, it appears larger than by direct rays. 



For the object is feen under the greateft angle, by thofe 

 rays that are moft refracted. 



Cor. 2. When an object is viewed by oblique rays 

 through fpeclacles, it is feen more diftinclly with one eye, 

 than with the other. 



For 



