33*2 



MICROSCOPE. 



will be caused to meet at a point, called the 

 focus, some distance beyond the centre of cur- 

 vature. This effect will not be materially 

 changed, by allowing the rays to pass into air 

 again through a plane surface of glass, such as 

 would be formed by a section of the glass in the 

 vertical line; a lens of this description is called 

 a plano-convex lens ; and it will hereafter be 

 shown to possess properties, which render it 

 very useful in the construction of microscopes. 

 But if, instead of passing through a plane sur- 

 face, the rays re-enter the air through a convex 

 surface, they will be made to converge still more. 

 This may be best understood by considering 

 the course of parallel rays, as in the adjoining 



Fig. 145. 



A B, parallel rays passing through glass and falling 

 upon, the convex surface F B G; B H, B H, 

 radii prolonged, which are the perpendiculars to 

 the curved surface at the several points ; B C, 

 course of the rays if unrefracted ; B E, their 

 course in consequence of refraction. 



figure (fig. 145). Here the radii prolonged will 

 be the perpendiculars to the curved surface; and, 

 according to the law of refraction just alluded to, 

 the rays passing from the dense into the rare 

 medium will be bent from the perpendicular, 

 so as to be made to converge towards a focus, 

 as in the former instance. It is easy to see, 

 therefore, that the effect of the second convex 

 surface will be precisely equivalent to that of 

 the first; for the contrary direction of the sur- 

 face is antagonized by the contrary direction of 

 the refraction ; so that the focus of a double 

 convex lens will be at just half the distance 

 from it, or (as commonly expressed) be half 

 the length of the focus of a plano-convex lens. 

 In fact, the focus of the former to parallel rays 

 will be the centre of its sphere of curvature, and 

 its focal length will therefore be the radius; 

 whilst the focus of the latter will be in the op- 

 posite side of the sphere, and its focal length 

 will be the diameter. Now it is evident that 



Jig. 146. 



Parallel rays fulling on a double convex lens brought to 

 a focus in its centre ; and rays diverging from Mich a 

 point rendered parallel. 



Fig. 147. 



Parallel rays falling on a plano-convex lens brottght to 

 a focus at the distance of its diameter, arid vice 

 versa. 



if a double convex lens will bring parallel rays 

 to a focus in the centre of its sphere of curva- 

 ture, it will on the other hand cause rays to 

 assume a parallel direction, which are diverging 

 from its focus ; so that if a luminous body were 

 placed in that point, all its cone of rays, which 

 fell upon the surface of the lens, would pass 

 out in a cylindrical form. Again, if rays al- 

 ready converging fall upon a convex lens, they 



Fig. 



148. 



Rays already converging brought to a focus nearer than 

 the centre ; and rays diverging from such a point, 

 still diverging in a diminished degree. 



Fig. 149. 



Rays diverging from points more distant than the prin- 

 cipal focus on either side brottght to a focus be* 

 yond it. 



will be brought to a focus at a point nearer to 

 it than the focus for parallel rays (which is 

 called its principal focus) ; and, if they be di- 

 verging from a distant point, their focus will be 

 more distant than the principal focus. The 

 further be the point from which they diverge, 

 the more nearly will the rays approach the pa- 

 rallel direction ; until, at length, when the ob- 

 jects are very distant, their rays in effect become 

 parallel, and are brought to a focus in the 

 centre of the sphere. If they diverge from the 

 other extremity of the diameter of the sphere, 

 they will be brought to a focus at a correspond- 

 ing distance on the other side of the lens. On 

 the other hand, if they be diverging from a point 



