CONSTRUCTION OF THE MICROSCOPE. 



13 



form a part ; nor even the eye itself." Rays of light falling perpendi- 

 cularly upon a surface of glass or other transparent substance, pass 

 through without being bent from the original line of their direction. 

 Thus, if a ray pass from k perpendicularly to the surface of the piece of 

 glass at e (fig. 3), it will 

 go on to h in the right 

 line k e o g h. But if the 

 same ray be directed to 

 the surface e obliquely, as 

 from a, instead of passing 

 through in a direct line 

 to b in the direction a e 

 m b, it will be refracted 



to d, in a direction ap- 

 proaching nearer to the 

 perpendicular line k h. 

 The ray a e is termed 

 the ray of incidence, or 



the incident ray ; and the angle a e k which it makes with the perpen- 

 dicular k h is called the angle of incidence. That part of the ray 

 from e to d' passing through the transparent medium is called the 

 ray of refraction, or the refracted ray ; and the angle d e g which it 

 makes with the 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 re- 

 fracted ray e d as that was from the Hue 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 ; but in passing out is bent into a line parallel to, and 

 running in the same direction 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 g m measures the angle of incidence g e m, 

 and the arc g d the angle of refraction g e d, A line m o drawn from, 

 the point m perpendicular to h k is called the sine of the angle of 

 incidence, and the line d g the sine of the angle of refraction. From 

 the conclusions drawn from the principles of geometry, it has been 

 found, that in any particular transparent substance the sine of the 

 angle of incidence m o has always the same ratio to the sine d g of the 



