360 MifceUanea Curiofa. 



Another Example (hall be when a Hemi- 

 sphere is expofed to parallel Rays, that is, d 

 and f being infinite, and t = r, and after due 



Reduction the Theorem refults — — rzuf. 



mm — mn 



That is, in Glafs it is at | r, in Water at % r '•> 

 but if the Hemifphere were Diamant, it would 

 collect the Beams at i t? of the Radius beyond 

 the Center. 



Laftly, As to the Effect of turning the two 

 fides of a Lens towards an Object ; it is evident, 

 that if the thicknefs of the Lens be very fmall,fb as 

 that you neglect it, or account t == o ; then in 

 all Cafes the Focus of the fame Lens 9 to whatfb- 

 ever Beams, will be the fame, without any diffe- 

 rence upon the turning the Lens : But if you are 

 ib curious as to confider the thicknefs, (which is 

 feldom worth accounting for) in the Cafe of pa- 

 rallel Rays falling on a Piano- Convex of Glafs, 

 if the plain fide be towards the Object, t does 

 occafion no difference, but the focal diftance 

 /=ir. But when the Convex-fide is towards 

 the Objeft, it is contraSed to x r — f t, fb that 

 the Focus is nearer by f t. If the Lens be double 

 Convex, the difference is lefs ; if a Menifcut 9 

 greater. If the Convexity on both fides be equal, 

 the focal length is about \ t (horter than when 

 t = o. In a Menifcus the Concave-fide to- 

 wards the Cbjedfc increafes the focal Length, 

 but the Convex towards the Object diminifhes 

 it. A General Rule for the difference arifing 

 on turning the Lens 9 where the Focus is Affir- 

 mative, is this - r * f for double 



Convcxes of differing Spheres. But for Me* 



nifci 



