6ff On the Form of Spectacle- Glasses. 



and obsolete form of lens, called a tfieutsms, instead of tht? 

 common glasses, I beg leave, sir, to ofler through the samer 

 channel a few observations on his arguments ; and my opi- 

 nion w hy I do not consider the contrivance entitled to any 

 claim, fjthcr to novelty or improvement. 



WhitiT a printed book, or other object;, i* viewed through a 

 convex spectacle-glass, or other lens, of a short focal di- 

 stance, such as seven inches down to fonriuchc?, or less, the 

 indistinctness observed of the surrounding parts, when the 

 central appears clear, arises from the spherical iiOTre of the 

 iens, and 19, by opticians, called the longitudinal aberration 

 of tlie knjt. There is another kind of al)crration connected 

 with this lens, called the lateral aberration, which is occa- 

 sioned by the prismatic form of the lens, producing a dif- 

 ferent refrangibility of the rays of light, and blending the 

 prismatic colours with their appearance of the object. It is 

 the longitudinal aberration only that I have now occasion 

 to consider. This aberration in lenses of the same foci in- 

 creases with their diameter and thickness, and of the same 

 diameter, is, in the inverse proportion to the foci. 



The ravs issuiuii from distant objects, arc more parallel to 

 each other, wheii incident upon the leng, than proximate 

 ones} therefore the aberration will be less. 



Hence it may be inferred, that when spectacle-glasses are 

 made larger in diameter than the angular extent of objects 

 to be seen through them require (a person in that case, 

 vithout much inclining the axis of his eyes, or feeling it in- 

 convenient, moving his head a little), they have very properly 

 been reduced somewhat in diameter, the aberration being 

 diminished, and consequently the objection, in a great de- 

 gree, removed, except in glasses that are of very short foci. 



In concave glasses the aberraticm, or indistinctness, is of 

 a similar nature; the defect of these l>eing from the imper- 

 fect divergence of the rays, instead of the imperfect conver- 

 ijcnce by convex glasses. 



Spectacle-glasses are now generally made of the double 

 concave and double convex kinds, or nearly so ; for a little 

 alteration of figure does not aftect the general appearance of 

 objects viewed through them. It is in science as in other cases, 

 that a general utility does not always depend upon trifling 

 alterations. Spectacles are recorded to have been hiventod 

 about the year 1300, and from reading, and many years ex- 

 perience ill this small but invaluable article, I really do not 

 kuow that during the elapsed time, an opticAl instrument of 

 any kind whatsoever has undergone more innovation and 

 lilcmpts .-^t improvement. Of many to my knowledge I 



shall 



