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Learn the parts of a light microscope.
Parts of a Light Microscope
Description of Parts. As it is necessary for the student to become conversant with the terms of the various parts and to understand their use, we give an illustration with letters, and append a list giving the names. 
A. Base on Foot. This is the foundation of the instrument. It usually rests upon three points (or should do so) and is of such a weight that it keeps the instrument firm when it is in an upright or inclined position. The revolving plate, when this is provided, by means of which the upper portion of the instrument is revolved, without changing the position of the base, is considered a part of it. B. Pillar. It is that portion which is fastened to the base and may be one or two, according to the construction of the stand. It carries upon its upper end the joint or axis. C. Arm. This is connected with the pillar by the joint and supports all theworhing parts of the instrument. D. Body. This is the tube-portion to which the optical parts are attached. E. Nose-Piece. This is an extra piece which is attached to the lower part of the tube. Society Screw. This is a standard screw which is cut into the nose-piece, and is called so from the fact that it was first established by the Royal Microscopical Society of London. It is also called the versa screw, and is in general use in this country and England ; it has lately been adopted by some firms on the Continent of Europe. F. Objectives. This is screwed into the nosepiece and is called so because it is nearest the object. It is the most important of the two optical parts (of the microscope proper) and upon its perfection the distinctness of the image and therefore the value of the instrument almost entirely depends. G. Eye-Piece or Ocular. It is called so because it is nearest the eye and is the remaining optical part. It magnifies the image given by the objective. This and objective will be treated more fully later on. H. Draw-tube. This is that portion of the body which moves in the outer sheath and which receives the eye-piece. It is provided for the purpose of attaining different lengths, variations in magnifying power and as a matter of convenience while working. I. Collar. This is a ring which is attached to the draw-tube and is usually provided with a milled edge. J. Coarse Adjustment. This is a provision for moving the body quickly back and forth for adjusting the focus approximately. It is done by a sliding rack and stationary pinion (not shown in cut ) or a sliding body in an outer sheath. K. Milled Heads. These are attached to the shank of the pinion, which is revolved by means of them and are usually large to give sensitiveness to the movement. L. Fine Adjustment. This is slow moving and serves to get an exact focus. It is attained by a fine thread, provided with a milled head, and acts upon the body, either directly or by levers. This as well as the coarse adjustment should be extremely sensitive and should not have the least side or lateral motion. The fact that either of them have it, is evidence of poor workmanship. M. Stage. This is the portion on which the object is placed for examination and is attached to the arm. N. Clips. These are two springs which are attached to the upper surface of the stage and serve to hold down the object. Centering Screws. These are provided for moving the stage in different directions to bring the center of its revolving motion in the center of the field. O. Mirror. This is used for reflecting and condensing light upon the object. As a rule two mirrors are used, one plane and the other concave. The first gives a comparatively weak light, while the second concentrates it and gives it more intensity. P. Mirror-bar. This carries the mirror and by a sliding arrangement allows the variations in distance of the mirror to the stage_; it also swings in a circle around the object in order to illuminate it from any direction. Q. Sub-Stage. This is a ring below the stage to receive various accessories which may be required. It is sometimes fixed to the stage but in the best instruments it is separated from it and is provided with an adjustment to vary its distance from the object. R. Sub-Stage bar. This receives the sub-stage and permits its adjustment. In modern American instruments this, as well as the mirror-bar, is on an axis in the plane of the stage, so that whatever position they may be in, relative to the object, the distance from this to the sub-stage or mirror does not vary, except when made to do so. S. Diaphragm. This is a perforated, revolving disk, attached either to the stage or sub-stage. It has holes of different sizes so that light from the mirror may be modified.
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