292 ON SPECTACLES. 



have taken place in their form, but without producing any bet- 

 ter optical effefl ; and this may occafion a fufpicion, that the 

 properties of the glafles made ufe of^ have not been clearly 

 underftood. 

 General pofition Emerfon obferves, that *' when glalTes are put into frames for 

 ou^hfto falf ^pe<^acles, thefe frames ought not to be flraight, fo as both 

 perpendicularly glafles may lie in the fame plane; but they ought to be fo bent 

 oi> the lens. in the middle, that the axis of both glafles may be dire6led to 

 one point, at fuch a diftance as you generally look with fpec- 

 tacles. By this means the rays will fall perpendicularly upon 

 both glafles, and make the obje£l appear diflind. But if they 

 fall obliquely upon the glafl^es, it will caufe a confufed appear* 

 ance in the objed */' Martin alfo bent the frames of his vifual 

 glafles, for the fame purpofe as above defcribed. And it ap- 

 pears from Dr. Wollafton's defcription of his Perifcopic SpeSia- 

 cles, that they are alfo conftruded on a principle to avoid ob- 

 lique rays f . 

 Affertlon: ^^ ^^ ^^^-^ ^^g'^^^''* ^^^^ this mode of conftrufling fpe6iacles 



that the perpen- (hould have been recommended by fo many writers on optics, 

 kaftada'Td*'^ when it is eafily proved, that the rays of light which fall per- 

 to diftlnft pendicuJarly upon the centre of a lens, enter the eye placed 



vifion. near it, in a more confufed flate than thofe rays which fall upon 



^ . a lens obliquely. 



General obs. ^'"« Smith, in his complete fyftem of optics, gives us this 



of Dr. Smith, general obfervation on vifion. " That the apparent diftind- 

 nefs and confufion of an objed depends upon the mutual in- 

 clination of the rays to each other in any one pencil when they 

 fall upon the eye."* 

 Illuftration j This obfervation being admitted, let L L, Fig, 1. reprefent 



by which his a double convex lens, a a, a ray of light pafllng perpendicu- 

 a pencil of rays^ larly through its centre; en, and dn, two other rays nearly 

 paifiHg near the parallel to the former. The ray a o fuffers no refraction j but 

 wi'l^ afford^vWion ^^^ ^^)' c n by paflSng through the lens, will be refraded to- 

 lefs diftinft wards the right hand, and the ray d n, by refraction, will be 

 turned towards the left ; confequently thefe rays have not a 

 mutual inclination to each other when they fall upon the eye 

 placed near the lens, as at x. 



* Emerfon's Optics, page 167. 



t Philofophical Journal, V. VII. p. 143. 



X Elementary parts of Smith's Optics, page 76. 



4 But, 



