194, 



The original 

 form moft ap- 

 proved. 



Theorem of 



Huygens con- 

 jfideied as 

 unlverfally ap< 

 pUcabl^ 



Eye glaffes of 

 fpe£tacles. 



Remarks on 

 Jfcnfes ; to af- 

 certain the 

 value of Dr. 

 Wollafton's 

 patent fpec- 



REMARKS ON SPECTACLES. 



attempts at improvement. Of many within my knowledge^ 

 I (hall only (ele€t the following as entitled to any degree of 

 commendation. /Ij/fcough's crown glafs fpeflacles. The bi- 

 fe6ted glaflfes of Dr. Franklin. Rihright's double glalTes. The 

 vifual glafl'es of that learned Optician Mr. Martin. The fquare 

 convex form by Siour. The patent combined glalfes of 

 Meflrs. Watkins and Smitli, injudicioufly called achromatic, 

 confifring of a convex lens, combined with a menifcus or con- 

 cave convex lens. In tlie various mountings of the fram«sS 

 there is a flill greater variety. 



Notwithrtanding Ihefe contrivances, univerfal experience 

 has caufed the original and iimple form of glalfes to fuperfedo 

 them; and it affords an indubitable proof, that it is the belt 

 and moft converiient that can be dcvifcd, when clear glals, 

 accurate tools, and good workmanOiip are ufed. 



The theorem given by Ilui/gemj and demonftrated by many 

 other fubfequent writers on optics, proving that a convex lens 

 having its radii of cuivature in the proportion of one to fix, 

 has lefs aberration than any other form of lens, when the 

 greateft convexity is towards the obje6l; and the fame for 

 the concave lens ; mud hold true fpr any ufe whatfoever, for 

 which fuch a ibrmed lens may be required. 



It does not appear to have occurred to Dr. W. that the 

 eye glalTes ufed to magnify the images formed by the object 

 glaifes in teiefcopes are of the beft form, when with the curves 

 of the proportion above mentioned. In the eye pieces of the 

 beft achromatic teiefcopes, they are always applied, and in 

 high powers, the image frequently fubtends an angle from the 

 centre of the eye glafs of fixty degrees or more. I have 

 never feen any corre^i dioptrical theorem, that tended to prove 

 that a menifcus, fingly, or combined, will anfwer fo per- 

 fectly the fame purpofe. 



The ordinary purpofes of vifion, are very well anfwered 

 by the common glafl'es, under an angle as large as eighty or 

 ninety degrees, and the beft artifts, or draughtfmen allow^ 

 that 60^ is as much as a fixed pofilion of the eye ought in 

 perfpective to embrace, to convey a juft reprcfentation on 

 the optic nerve. 



To perfons, the humours of whofe eyes are fo decayed as 

 lo be deficient in their original refradlive power, glalfes of fliort 



i'oci 



