On the Form of Spectacle-Glasses, 67 



shall only select the following as entitled to any degree of 

 commendation — ^i/scough's crown-gluss spectacles, the bi- 

 sected glasses of Dr. Franklin, the visual glasses of that 

 learned optician Mr. Martin, the square convex form by 

 Storer, the patent combined glasses of Messrs. Watklns and 

 Smith, injudiciously called Achromatic, consisting of a coij- 

 vex lens combined with a meniscus or concavo-convex lens. 

 In the mountings of the frames a still greater variety could 

 be enumerated. 



Notwithstanding these contrivances, universal experience 

 has caused the original and simple form of glasses to super- 

 sede them ; and it affords an indubitable proof that it is the 

 best and most convenient that can be contrived, when clear 

 glass, accurate tools, and good workmanship, are used. The 

 theorem given by Huygens, and demonstrated by many 

 other subsequent writers on optics, proving that a convex 

 lens, having its radii of curvatures in the proportion of one 

 to six, contains less aberration than any other form of lens, 

 when the greatest convexity is towards the object ; and the 

 sauic for tiie concave lens must hold true for any use whatso- 

 ever for which such a formed lens may be re(|uired. 



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

 eye-glasses used to magnify the images formed by the object- 

 glasses in telescopes are of the best form, when with 

 the curves of the proportion above mentioned. In the eye- 

 pieces of the best achromatic telescopes they are always ap- 

 plied, and, in high powers, the image frequently subtends an 

 angle from the centre of the eye-glass of sixty degrees or 

 more. I have never seen any correct dioptrical theorem that 

 tended to prove tliat a meniscus, singly or combined, will 

 answ er so perfectly the same purpose. The ordinary pur- 

 poses of vision are very well answered by the common 

 glasses under an angle as large as eighty or ninety degrees ; 

 and the best artists or draughtsmen allow, that Qif is as nuich 

 as a fixed position of the eye ought in perspective to embrace, 

 to convey a just representation on the optic nerve. 



To persons the humours of whose eyes are so decayed as to 

 be deficient in the original refractive power, glasses of short 

 foci will to them render the extreme parts of objects some- 

 what confused, but in a nmch less degree than to persons 

 with perfect eyes orundecayed humours. 



In telescopes and microscopes the aberration is usually cut 

 off by the insertion of circular apertures or sto])s, but iii 

 spectacles this is not essentially necessary, nor does tlie 

 want of them, nor the figure of the glasses, prove that they 

 are conclitutionally bad ami prejudicial. 



E2 Thq 



