REMARKS ON SPECTACLES. 193 



When a printed book, or other obieft, is received through Aberration of 

 ro, . r , , r\ r ..-n light refrafted 



a convex Ipectacle-glais, or other lensot a Ihort local diltance, ^y lenf«s. 



fuch as from feven inches down to four inches or lefs, the in- 

 diftinflnefs obferved of the furrounding parts, when the central 

 appear diftincl, arifes from the fpherical figure of the lens, 

 and is by Opticians called the longitudinal aberration of the 

 lens. There is another kind of aberration connected with this 

 lens, called the lateral aberration, which is occafioned by the 

 prifraatic form of the lens, producing different refrangibility 

 of the rays of light, and blending the prifmatic colours with 

 the appearance of the obje6l. It is the longitudinal aberra- 

 tion only that I have now occafion to confider. This aberra- 

 tion in lenfes of the Time foci, increafes with the diameter 

 and thicknefs; and of the fame diameter is in the inverfe pra« 

 portion to the foci. 



The rays ilTuing from diftant objecls, are more parallel to 

 each other when incident upon the lens, than thofe from 

 proximate ones; therefore the aberration will be lefs. 



Hence, it may be inferred, tliat when fpedtacle-glafles have Contradlng th« 

 been made larger in diameter than is fufficient to obferve i'^^^"~J f. 

 through, the angular extent of obje61s have very properly been approved. 

 fomewhat reduced in diameter, fo that a perfon might ufe 

 them without much inclining the axis of his eyes, or finding it 

 inconvenient or unnatural to move his head a little. For the 

 aberration is thus diminiflied, and confequenfly the objedlion 

 in a great degree removed, except when the glafles are of ; • ' ;• 

 very lliort foci. 



In concave glafles, the aberration or indiftindnefs is of a Concaves, 

 fimilar nature ; the defe6l of thefe being from the imperfect 

 divergence of the rays, inftead of the imperfect convergence 

 in convex glalfes. 



Spedlacle-glafTes are now generally made of the double Remarks on 



concave, and double convex forms or nearly fo : for a little !^P^'^^*^l*^V".^ . 

 1. .. ^ f. , ^ ^ , , fo"i2 ot *he im* 



alteration ot hgure does not affed the general appearance of provements 



objedts viewed through thenj. It is in fcience, as in other "^^'^^ ^" ^*^« ^* 



cafes, that general utility does not always depend on trifling 



alterations. Spedacles are recorded to have been invented 



about the year 1300, and from much reading and many years 



experience in this fmall, but invaluable article, I really do not 



know, that during the elapfed time, any optical inflrument ' y 



of any kind whatfoever has undergone more innovation, and 



Vol. VII. —March, ISOI. Q attempti 



