212 Magnifying Glajes. [Book lit. 



mufl necefftrily have the effect of magnifying glades, 

 fmce by the convergence of the rays the vilual angle is 

 rendered more obtufe, and confequently the image 

 which is depicted on the retina muft be proportionably 

 larger. 



The mode of rinding, upon mathematical principles, 

 the focus of parallel rays or principal focus in thefe 

 glades, will be explained in a fucceeding chapter!} and 

 it may be eaiily found, though not with equal exact- 

 nefs, by holding a iheet of paper before the glafs when 

 expofed to the rays of the fun, and obfcrving the dif- 

 tance of the paper from the glafs, when the luminous 

 fpot on the paper is very final), and when it begins to 

 burn ; or when the focal length does not exceed three 

 feet, the focus may be found by holding the lens at 

 fuch a diftance from the wall oppofite a window fafli, 

 that the image of the fafh may appear diftinct upon 

 the wall. The principal focus, or as it is often called 

 the focus, of a double convex lens is at the length 

 of the radius, or fernidiameter of that circle which is 

 formed by the convexity of cither of its furfaces. 



From this property in convex Icnfes of rendering all 

 rays in fome degree convergent which fall upon their 

 furfaces, it is evident that in all fuch cafes there muft: 

 be a point or focus, where rays proceeding from the 

 .extreme point of any object muft crofs each other; and 

 confequently an inverted image of the object will be 

 exhibited at any diftance beyond that point. This 

 may be elucidated by a very eafy experiment, viz. by 

 holding a common reading or magnifying glafs be- 

 tween a candle and a flieet of paper fufpended on the 

 wall, at a proper diftance, when the image of the 

 candle will appear on the paper inverted j and the rea- 

 . fon of this is extremely clear, for it is evident that the 

 upper rays, after refraction, are thofe which proceeded 



from 



