EXERCISE 17 



The compound microscope is a very delicate instrument, embodying a great deal of scientific knowl- 

 edge and mechanical skill. It should be handled with intelligence and consideration. 



Problem. What are the parts of the microscope, and how should it be used ? 



What to use. A compound microscope ; a prepared slide. 



What to do. 1. Study the microscope as you read the description of its structure below. 



2. Study the specimen on the prepared slide, following the directions carefully. 



Record. 1. On the left-hand side, label all the parts of the microscope picture with the names 

 that are underlined in the description. On the right-hand side, tell the use, or function, of each part 

 that has been labeled. 



2. Make a drawing of the object studied under the low power of the microscope, as large as it 

 appears. 



NOTE. I. There are three distinct sets of parts in a compound microscope: (a) the mechanical, which support the 

 other parts and make possible their controlled movements ; (b] the optical, which actually do the magnifying by their effect 

 upon the rays of light passing through them ; and (c) the illuminating, which direct and regulate the light supply. 



2. All of the mechanical parts together constitute the stand, which has a heavy base, supporting a leg, or pillar. Pro- 

 jecting horizontally from the top of the pillar, parallel with the base, is the flat stage, with a hole in the middle for letting 

 light through. Above the stage is a continuation of the pillar, and extending from this over the stage is the arm, which 

 carries a vertical tube. Projecting on both sides of the arm are knobs with rough edges. These are connected with a pinion 

 that makes the tube go up or down when they are turned. The knobs, with the rack and pinion, make up the " coarse 

 adjustment," and the knobs are sometimes called the coarse adjustment or coarse-adjustment screws. By turning them gently 

 (one at a time) you can see the effect on the tube. The coarse adjustment is used to change the distance between the lenses 

 and the object to be observed ; that is, to focus. At the very top of the pillar is another screw, the fine adjustment, which is 

 also used for focusing but is much more delicate than the coarse adjustment. 



3. The optical system consists of two sets of lenses in metal cases. The set placed at the lower end of the tube, near 

 the object, is called the objective. On some microscopes there is a special attachment at the base of the tube for carrying 

 two or more objectives conveniently ; this is the nosepiece. With the use of the nosepiece it is possible to change from 

 one objective to another with no loss of time. The objective that is in line with the tube would be the one in use. Where 

 there are two or more objectives, the longer or longest is of greater magnifying power. At the other end of the tube, near 

 the eye, is a set of lenses in a metal case, called the eyepiece, or ocular. This is easily taken out of the tube. 



4. The illuminating system consists of the mirror, hung under the stage, and of the diaphragm, inserted in the 

 opening of the stage. The mirror usually has two faces, one flat and one concave. It can be turned in all directions and is 

 used for throwing a beam of light from the window (or a suitable lamp) up through the object resting on the stage, through 

 the objective and through the ocular, into the eye. The diaphragm is .an arrangement for enlarging .or diminishing the 

 amount of light coming through, by making the opening larger or smaller. 



5. Some microscopes have a joint in the pillar, just below the stage, permitting the upper part of- the stand to be 

 tilted into a more convenient position. On some microscopes one or two clips on the upper surface of the stage hold the 

 slide in place. 



CAUTION. In lifting or carrying the microscope, grasp it firmly around the pillar under the stage, 

 unless there is a special handle for grasping above the stage. 



Allow nothing to touch any of the optical parts except specially prepared lens paper or a clean 

 linen handkerchief. 



Questions. 1. In which direction must the coarse adjustment be turned to raise the tube? 



2. In which direction must the fine adiustment be turned to raise the tube ? 



Using the coarse adjustment, raise or lower the tube until the tip of the low-power objective is 

 about half an inch above the stage. 



Looking through the eyepiece, turn the mirror until you have an even white field of light. 



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