28 PRACTICAL HISTOLOGY. 



an objective. Generally speaking, it is well to keep to eye- 

 pieces of medium power, such as the B or C of English 

 makers and the No. 3 of Hartnack. 



41. Distance of the Ocular from the Objective. 

 The magnifying power of the compound microscope may 



be varied by altering the distance between the objective 

 and the ocular. Some microscopes are so constructed that 

 this is impossible. Nevertheless it is extremely convenient 

 to have the tube between the lens and the ocular made 

 like that of a telescope (Fig. i). One not unimportant 

 advantage is, that such a microscope can be packed in a 

 comparatively small box. The great advantage, however, 

 is, that by varying. the length of the tube very fine gradations 

 of power can be obtained with the same lens and ocular. 

 In measuring an object with an eye-piece micrometer it 

 will afterwards be seen how important this is. The tube 

 when fully elongated should not, however, reach more than 

 ten inches above the stage, for it is often important in 

 drawing and in measuring the magnifying power of the 

 microscope to have the surface of the stage in the focus of 

 the eye that is not looking through the microscope. 



42. The Bull's Eye Condenser. Opaque objects are 

 seen by reflected light. The light is concentrated upon them 

 with an ordinary condenser consisting of a plano-convex 

 lens (c, Fig. i). This may be fixed to a separate support, 

 but in an ordinary microscope there is no place for it so 

 convenient as that shown in Fig. i. Considerable experi- 

 ence is required in the use of this apparently simple 

 apparatus. It is important to bear in mind that the 

 condenser should always, if possible, be placed at right 

 angles to the direction of the light, and that, with a covered 

 object, the light should fall on the cover-glass as directly 

 as possible, for if it be very oblique it is to a large extent 

 lost by reflection from the cover. 



43. The Mirror. Transparent objects are illuminated 

 by transmitted light reflected from a mirror (N, Fig. i). 

 The mirror is usually slightly concave, and its distance 

 from the object should be capable of alteration by a sliding 

 or jointed movement, so that when working with a low 



