42 THE MICROSCOPE. 



or claw horseshoe shaped, that it may steadily grasp 

 the table and assist in maintaining the centre of gravity 

 in any position the instrument may be placed. To 

 the foot should be attached two upright pillars, with 

 trunnions, for making the attachments and securing 

 the sliding bar which carries the tubular body and 

 stage. 



The tubular body should be about eight or ten 

 inches in length, with a second or inner tube, "the 

 draw-tube," of five or six inches in length, sliding with- 

 in it. The "draw-tube" assists in the magnification 

 of the image, and is usually engraved with a scale of 

 inches and parts of an inch, for measuring the dis- 

 tance between the eye-piece and the front of the 

 object-glass. The eye-piece surmounts the body, while 

 to the other extremity is attached or screwed the 

 object-glass. The focus is obtained by either a sliding 

 or rack-and-pinioii motion, termed the " coarse-ad- 

 justment," the fine-adjustment being obtained by a 

 milled-head screw acting upon the long end of a lever, 

 or by other mechanical means. Whatever the kind of 

 motion adopted, neither jumping nor lateral movement 

 of the body should take place, otherwise the object, 

 when placed on the stage, will appear to change its 

 position either to the right or left. A fine-adjustment 

 is very necessary, as without it the highest magnifying 

 powers can hardly be used without risk of damage 

 being done either to the object or the objective. 



Pig. 25 represents the body of an ordinary com- 

 pound microscope with triple object-glass ; o is an 

 object, above it is seen the triple achromatic object- 

 glass, in connection with the eye-piece e e, ff the plano- 

 convex lens ; e e being the eye-glass, and // the field- 

 glass, and between them, at b &, a dark spot or dia- 

 phragm. The course of the light is shown by three 

 rays drawn from the centre, and three from each end 

 of the object o ; these rays, if not prevented by the 

 lens / /, or the diaphragm at b &, would form an image 

 at a a ; but as they meet with the lens f f in their 

 passage, they are converged by it, and meeting at b b, 

 where a diaphragm is placed to intercept all extraneous 



