CH. XIV] IMAGE FORMATION, PROJECTION MICROSCOPE 599 



tion to give a sharp image on the screen. To accomplish this best, 

 the objective is so placed that the focus of the condenser is at the 

 center of the projection objective, and then the stage of the micro- 

 scope is moved back and forth until the image is sharp upon the 

 screen. If a three-lens condenser, without a substage condenser is 

 used, it will be found best for low powers (20-125 mm. focus) to 

 have a condensing lens next the objective (2d element of the con- 

 denser, fig. 332) of 20 to 25 cm. (8 to 10 in.) principal focus. For 

 the higher powers where greater numerical aperture is needed, a 

 condenser lens of 15 cm. focus is better. 



FIG. 338. IMAGE FORMATION WITH AN AMPLIFIER. 



O The back lens of the objective. 



A Amplifier (divergent lens). 



/' The image which would be projected by the objective if no amplifier were 

 in place. 



/ Image projected with the amplifier in place. 



Note that the rays from A diverge more rapidly than from making the 

 image larger than without the amplifier. (See also fig. 126). 



834. Illumination for high powers. In all high power micro- 

 scopic projection (2 to 16 mm. objectives) any source of light should 

 be considered as an extended source whether lime light, arc light, 

 or the sun is used. 



The best method of illuminating microscopic specimens has been 

 found to be to place the microscope so that the front lens of the 

 objective is in the image of the crater (or the sun), (fig. 140) and 

 then the specimen is moved up toward the objective until its image 

 is in focus on the screen. Light will extend from every point of the 

 object as shown in fig. 347 and strike the front lens of the objective. 

 The action of the objective is to bring all of the light leaving a point 

 of the object to a single point on the screen. The details of image 



