

OPTICAL INSTRUMENTS. 



425 



or more convex lenses placed in a tube. One of these, o, 

 called the object-glass or objective, is of short focus. The 

 object, ab y being placed slightly beyond the principal focus, 

 a real image, ce?, magnified and inverted, is formed within 

 the tube ( 630). The other lens, E, called' the eye-glass, 

 is so placed that the image formed by the objective lies 

 between the eye-glass and its focus. A magnified virtual 

 image, AB, of the real image is formed by the eye-glass 

 ( 631) and seen by the observer. (See Fig. 328.) 



(a.) Compound microscopes are usually provided with several 

 objectives of different focal distances, so that a selection may be 

 made according to the magnifying power required. The powers 

 generally used range from 50 to 350 diameters (i. e., they multiply 

 linear dimensions so many timesX The object generally needs to 

 be intensely illuminated by a concave mirror or convex lens. 



FIG. 329. 



66O. Galilean Telescope; Opera Glass. In 



the telescope attributed to Galileo the objective is a double 

 convex, and the eye-piece a double concave lens. The 

 concave lens intercepts the rays before they have reached 

 the focus of the objective ; were it not for this eye-piece, a 

 real, inverted image would be formed back of the position 

 of the concave lens. The rays from A, converging after 

 refraction by 0, are rendered diverging by (7; they seem to 

 diverge from a. In like manner, the image of B is formed 

 at b. The image ab is erect and very near. An opera- 

 glass consists of two Galilean telescopes placed side by 

 side. In a good instrument both lenses are achromatic. 



