COMPOUND MICROSCOPE. 



21 



13 inches as average) to the focal length of the lens. It will 

 be seen that magnification is greater as the focal length is 

 smaller and as the observer's distance of distinct vision is 

 greater. 



COMPOUND MICROSCOPE 



The simplest form would consist of two simple microscopes or 

 magnifiers, one with short focus, placed near the object, called 

 the objective, the other with longer focus, placed next the eye, 

 and called the eye-piece or power. The objective forms an in- 

 verted real image of the object, and, by means of the eye-piece, 

 we see a virtual, erect, magnified image of the real image. 



Fig. 21. 



eye-piece 



MODE OF ACTION. In Fig. 21, AB is an object; an image is 

 formed at ab, real, inverted and magnified. The eye-piece 

 forms an imaginary, erect, magnified image of ab at A'B'. This 

 is the principle of all compound microscopes. This form would 

 be very defective on account of spherical and chromatic aberra- 

 tions, and we will now study the more perfect microscope. 



DESCRIPTION OF COMPOUND MICROSCOPE. Fig. 22, A is the 

 base; B, pillar; C, pillar and arm; D, body; E, nose-piece; F, 

 objective; G, ocular; H, draw-tube; I, collar; J, rack and pin- 

 ion ; K, coarse adjustment ; L, fine adjustment ; N, spring clips ; 

 O, mirror; P, mirror bar; Q, diaphragm and substage; R, sub- 

 stage screw; S, stage; T, pillar hinge-joint. 



Only a few words need be said about the mechanical parts of 

 the instrument, as the figure will explain sufficiently. 



By the rack and pinion movement K, the body I) is given a 

 large up-and-down motion and a body is quickly brought into 

 rough focus. Then by the micrometer screw L the fine adjust- 

 ment is made, a very small motion of the body D being pro- 

 duced by one turn of the screw. 



The d'raw tube H carries a scale so that any tube length can 

 be obtained by pulling out or pushing in. The ocular G slips 



