14 CONSTRUCTION OF THE MICROSCOPE. 



angle of refraction, no matter what be the degree of obliquity with 

 which the ray of incidence a e is projected to the surface of the trans- 

 parent medium. If the ray of incidence passes from air obliquely into 

 water, the sine of incidence is to that of refraction as 4 to 3 ; if it passes 

 from air into glass, the proportion is as 3 to 2 ; and if from air into 

 diamond, it is as 5 to 2. 



By the help of glasses of certain forms, we unite in the same sensible 

 point a great number of rays proceeding from one point of an object ; 

 and as each ray carries with it the image of the point from whence it 

 proceeded, and all the rays united must form an image of the object 

 from whence they were emitted, this image is higher in proportion as 

 there are more rays united, and more distinct in proportion as the 

 order in which they proceeded is better preserved in their union. The 

 point at which the object must be placed is called the focus of the lens; 

 and the distance from the middle of the lens to the focus is called the 

 focal length, or distance. In every lens the right line perpendicular to 

 the two surfaces is called the axis of the lens, and is seen in the annexed 

 figure ; the point where the axis cuts the surface is called the vertex of 

 the lens ; the middle point between them the centre ; and the distance 

 between them the diameter. 



Fig. 4 is intended to represent the different forms of lenses in use : 



a is a plane glass of equal 

 thickness throughout ; b 

 a meniscus, concave on 

 one side, convex on the 

 other; c, a double-con- 

 cave j d, a plano-concave ; 

 e, a double-convex; f } a 

 / plano-convex. 



Refraction of Light through Lenses, and Method of tracing the 

 Progress of Rays. 



"When a ray of light enters the concave surface of a dense medium, 

 or quits a similar surface and enters the convex surface of a rare 

 medium, the method of tracing its progress is shown in fig. 5, where m'n 

 is a dense medium of glass with two concave surfaces, forming a thick con- 

 cave lens. Let c c be the centres of the two surfaces lying in the axis 

 c c', and h' r, h"r parallel rays incident on the first surface. As c r is per- 



