u 



CONSTRUCTION OF THE MICROSCOPE. 



In like manner, a concavo-convex lens, fig. \\,ll', whose 

 wncave surface I a V is a circle described round the farther 



Fig. 11. 



focus of the ellipse, will cause parallel rays b I, b' I , to 

 diverge in directions I r, I' r", which, when continued back- 

 wards, will meet exactly in the focus f, which will be its 

 virtual focus. 



If a plano-convex lens, fig. 12, has its convex surface 

 I a V part of a hyperboloid, formed by the revolution of a 



Fig. 12. 



hyperbola whose greater axis is to the distance between 

 the foci as unity is to the index of refraction, then parallel 

 rays rl, r" V falling perpendicularly on the plane surface, 

 will be refracted without aberration to the further focus 

 of the hyperboloid. The same property belongs to a 

 plano-concave lens having a similar hyperbolic surface, and 

 receiving parallel rays on its plane surface. 1 



(1) It must be borne in mind, that in none of those lenses would the object 

 be correctly seen in focus, except at the one point known as the mathematical 

 or geometrica 1 axis of the lens. 



