CONSTRUCTION OF THE MICROSCOPE. 17 



perpendicular is called the angle of refraction. The ray 

 projected from a to e and refracted to d, in passing out of 

 the transparent medium as at d, is as much bent from the 

 line of the refracted ray e d as that was from the line of 

 the original ray a e b ; the ray then passes from d to c, 

 parallel to the line of the original ray a e b. It follows, 

 then, that any ray passing through a transparent medium, 

 whose two surfaces, the one at which the ray enters, and 

 the one at which it passes out, are parallel planes, is first 

 refracted from its original course j but in passing out is 

 bent into a line parallel to, and running in the same direc- 

 tion as the original line, the only difference being, that its 

 course at this stage is shifted a little to one side of that of 

 the original. If from the centre e a circle be described 

 with any radius, as d e, the arc a a' measures the angle of 

 incidence a e Jc, and the arc g' d the angle of refraction 

 g e d. A line a Jc drawn from the point a perpendicular 

 co k h is called the sine of the angle of incidence; and the 

 line d g drawn from the point d perpendicular to Jc h is 

 called the sine of the angle of refraction. From the con- 

 clusions drawn from the principles of geometry, it has 

 been found, that in any particular transparent substance 

 the sine of the angle of incidence a k has always the same 

 ratio to the sine d g of the angle of refraction, no matter 

 what be the degree of obliquity with which the ray of inci- 

 dence a e is projected to the surface of the transparent 

 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 j 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 brighter in pro- 

 portion 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 

 parallel rays meet after converging through a lens is called 

 the principal focus, and its distance from the middle of 



