26 SELECTION AND USE 



Broad Gauge, is sometimes used in addition to the Society 

 Screw. 



In all microscopes, means are provided for moving the objec- 

 tive to and from the object, so as to bring the latter into focus, 

 as it is called. According as the device used for this purpose 

 acts coarsely but rapidly, or slowly but delicately, it is called 

 a coarse or a fine adjustment. The best microscopes are provided 

 with both kinds, so that the object is first brought approxi- 

 mately, but rapidly, into focus by one, and then adjusted more 

 slowly, but with great accuracy, by the other. 



The Coarse Adjustment has several forms. In Fig. 1 it consists 

 of a rack and pinion. In some cases it is a chain movement; 

 very often it is effected by merely sliding the body up and 

 down through a tubular collar by hand, as in Fig. 12. 



The Fine Adjustment usually consists of a fine screw, some- 

 times called, improperly, a micrometer screw, which moves either 

 the entire body or the lower part of it, called the nose-piece. 

 In some cheap stands, the fine adjustment is effected by moving 

 the stage towards the objective. 



The Nose-piece is a short tube, which fits into the lower end 

 of the body, and carries the Society screw at its lower end. 

 Sometimes it is made to slide out and in, and thus forms part 

 of the fine adjustment. In the instrument figured in the dia- 

 gram, it is immovable. The term nose-piece is also applied to 

 certain accessories which enable us to attach different pieces of 

 apparatus to the microscope, as, for example, two or more ob- 

 jectives at one time, the analyzer of the polariscope, etc. 



A Diaphragm is usually a thin plate of metal pierced with a 

 hole, the size of which regulates the diameter of the pencil of 

 rays that pass through. There is a diaphragm in the eye-piece 

 which contracts the field of view, and cuts off those rays which 

 tend to confuse the images, and all good microscopes have a dia- 

 phragm attached to the stage, whereby the amount of light 

 passing through the object to the objective may be regulated. 

 Instead of a thin plate of metal, an arrangement known as the 

 "Iris diaphragm " is used in the microscope shown in the 

 diagram. 



The Eye-piece or Ocular is the short brass tube, with its lenses, 

 which is next the eye. The eye-piece contains an Eye-Glass, 

 which is that next the eye ; a Field-Glass, placed next the 

 objective, and a Diaphragm, consisting of a brass plate with a 

 hole through it, and so arranged as to cut off the outer rays of 

 light. The tube in which these lenses are secured is in almost 

 all cases removable, and the best microscopes are furnished 



