GENERAL MiaiOWOl'H' TM'1L\1<JUK 



so-called Abbe's coml< n*<r is used. This is of especial service 

 when very high powers are used. 



The tube of the microscope can in most instruments be 

 moved up and down by two mechanisms, the coarse adjustment, 

 which consists of a rack and pinion ; and the fine adjustment, 

 which is made up of a micrometer screw at the upper end of the 

 upright column. By turning this to the right or left, the tube 

 may be lowered or raised by a fraction of a millimetre. 



The essential part of the microscope is the lens system. 

 There are two parts of this, the ocular (eye piece) and the 

 oltji'cfire. The ocular fits in the upper end of the tube, while 

 the objective screws into its lower end. The ocular is a 

 hollow cylinder with a lens at each end. The upper lens is the 

 ocular lens, the lower one, the collective It- /us. The objective 

 consists of a whole series of convex lenses (three or four), of 

 which the smallest one lying next the object to be examined is 

 called the front lens. We distinguish two kinds of objectives, 

 the dry lens and the oil-immersion lens. The first serves all 

 ordinary purposes ; while the latter is used in the study of finer 

 cellular and nuclear structure, as well as in bacteriological 

 study. The difference between these two kinds of lenses con- 

 sists in the presence of a layer of air between the dry lens and 

 the object ; while with the immersion lens there is interposed a 

 medium between the lens and the object, which has a refractive 

 index nearly equal to that of glass. This is of importance, 

 because the rays of light from the object must pass through the 

 cover glass into the air before they reach the objective. In 

 passing over from the glass to the air the rays are bent out- 

 ward, and a part of them thus do not enter the objective. By 

 using a denser medium than air this difficulty is obviated. 

 For this purpose cedar oil generally is used a drop is placed 

 on the cover glass over the object, and the tube lowered until 

 the front lens enters the oil. 



In recent years Karl Zciss in Jena has devised the so-called 

 apochromatic objective, by which the chromatic and spherical 

 aberration is overcome. 



A convenient but unessential addition to the stand is the 



