766 



COMPOUND MICROSCOPE. 



in the compound microscope enables us to use an object-glass of a 

 lower power than would otherwise be necessary. The compound 

 microscope, when well constructed, gives a flat and colourless picture 

 of the object, with clearness of definition. The observer can use it 

 for a length of time with less fatigue than when employing the simple 

 microscope. Weak eye-pieces and strong object-glasses are to be re- 

 commended. The eye-piece does not add either clearness or distinctness 

 to the object, and when it is very powerful the field of view becomes 

 too small to take in the whole image formed by the object-glass ; for 

 the magnitude of the field of view and the strength of the iUuminar 

 tion diminishes according to the magnifying power of the eye-piece 

 employed. The lower powers are of use in searching for the object 

 to be examined, which may thus be more easily found by a higher 

 power. For the lower power a linear amplification of from 20 to 50 

 diameters, and for a higher power a linear amplification of from 300 

 to 500 diameters at most, will give a sufficiently wide range of powers. 



The powers are increased 

 by a more powerful eye- 

 piece or object-glass, or by 

 both, or by lengthening 

 the tube of the microscope. 

 In examining vegetable 

 structures, an instrument 

 magnifying 150 to 200 

 diameters is usually suffi- 

 cient ; but in some instan- 

 ces higher powers are re- 

 quired. Achromatic object- 

 lenses of 1^, -I, and \ of an 

 inch focal length are recom- 

 mended as the most essen- 

 tial ; and two eye-pieces 

 should be provided, one of 

 about 1^ and the other of 

 2 1 inches in length. The 

 instrument shoidd have 

 both a coarse and a fine 

 adjustment ; and it is of 

 importance that it should 

 be made to incline or to 

 stand vertical. A movable 

 stage is also useful, and a 

 spring-holder to fix the objects on the stage, so that the different parts 

 of the object may be viewed without being touched by the fingers. 



Fig. 946. Ordinary compound microscope- 



Fig. 946. 



