16 PRACTICAL HISTOLOGY. 



should, when used with the ordinary eye-piece, give 

 an apparent linear enlargement of about 75 (75 dia- 

 meters); that is to say, when a line, the length of 

 which is known (say y-J-Q- of .an inch), is observed 

 through this combination, it should appear seventy- 

 five times as long as it really is (f inch* therefore). 

 The other, to be mentioned as the high power, should 

 give an apparent linear magnification of from 300 

 to 400 times. These two glasses amply suffice for 

 all ordinary histological studies; but for certain spe- 

 cial subjects it is advantageous to obtain the use of 

 a more powerful combination; one that will magnify 

 1000 diameters or more. 



Glasses of this high magnifying power are usually of 

 the kind known as "immersion-objectives," so-called be- 

 cause they are made so as to be used with a stratum of 

 water between the specimen and the lower lens of the 

 objective. The water (distilled) is applied to this lens 

 with a splinter of wood or from a pipette before the objec- 

 tive is screwed on to the tube of the microscope ; the tube 

 is then lowered by means of the coarse adjustment until 

 the drop of water comes in contact with the cover-glass, 

 after which the focus is obtained by cautiously lowering 

 it further, at first with the coarse, and then with the fine 

 adjustment. 



These comprise the essential requirements of a 

 microscope, but larger instruments are often pro- 

 vided with certain adjuncts which render them in a 

 measure more complete. Thus the stand may be 

 hinged so that the stage and tube can be tilted 

 somewhat out of the perpendicular to allow of better 

 adaptation to the position of the observer. But if 

 the microscope tube is not inconveniently high, it is 

 almost as comfortable to work without inclining the 

 instrument. Moreover, very many preparations, most 

 of those, for instance, which have to be examined in 

 fluid, will not admit of inclination. A camera lucida 

 (one which does not necessitate the tilting of the 

 microscope) is useful for obtaining an exact sketch 

 of the outlines of an object. 



