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Learn how to properly focus a light microscope.
Microscope Focusing
In every optical instrument the centre of the field gives the finest definition, anid the object being examined should be placed near the centre. An absolutely flat field is incompatible with the finest definition in the centre, and although in certain types of telescopes and photographic lenses the importance of a flat field is so great that a compromise is made, no deterioration of the central image can be allowed in the microscope. The penetration or depth of an object glass or the number of different layers of an object that can be seen sharply at the same time with a microscope is very small. With lenses of a high aperture, and therefore in general of a high magnifying power, the penetration decreases at a very rapid rate, and the power of seeing different planes sharply must depend on adjusting the instrument. It has been said that the depth of focus of a high-power microscope is really the fine focussing adjustment. The fine adjustment in the hands of a skilled observer is in constant motion, focussing first to one plane and then to another ; by this means a perception of depth is obtained which could never be given by an object glass fixed at one focus. The penetration of the microscope may be increased by inserting a stop with a small aperture immediately behind the object glass, but such a method reduces the aperture and consequently the detail that can be seen. It is seldom adopted except for photographing certain objects where the image from the upper or lower portion of the object obscures the layer being photographed, or for photographing objects with comparatively coarse structure. An iris diaphragm is made that will screw into the body of the microscope between it and the object glass for this purpose. There is only one position in relation to the lenses where an object can be placed to give a perfectly clear picture. This position is generally called the focus, and the microscope is said to be " in focus" when it is so adjusted that the object is in this position. It is more convenient to effect this adjustment by moving the body which carries the lenses rather than by moving the object. The coarse focussing adjustment is actuated by a helical rack and pinion which moves the body along a slide towards or away from the object. Turning the milled head so that its upper edge moves towards the observer, raises the body ; away from the observer, lowers it. It is a sufficiently delicate motion for focussing with object glasses of lower power than 1/6-inch (4 mm.).
The fine focussing adjustment does exactly the same as the Fine coarse adjustment, but the movement is far more delicate : adjustment, it is actuated by a micrometer screw and a lever moving the whole body along a second slide. A complete turn of the screw moves the body about a quarter of a millimetre. Turning the fine adjustment milled heads moves the body in the same direction as those of the coarse adjustment. In the " Standard " microscope the left-hand milled head is twice as delicate a motion as that on the right-hand side. The fine adjustment is required for the focussing of high powers and for examining the different layers of an object. In moving the body of the microscope up and down to obtain The best the correct focus, care is required to prevent the front of the object glass being forced into contact with the object by racking it too far down. It is easy to break a valuable specimen by this means; and although for its protection the metal mount of the object glass projects slightly in front of the front lens, it is delicate in construction, and can be damaged by being brought into contact with the specimen. Experienced microscopists can focus a lens downwards and stop at the position where the object is sharply seen, but it is unsafe. The correct method is to set the body of the microscope so that the front of the object glass almost but not quite touches the object, and then to rack backwards, turning the milled heads so that the upper portion turns towards the observer, and raise the body until the correct focus is found. With high-power object glasses, especially oil-immersion lenses, this method is not so easy because the distance of the correct focus may be below the point at which the body has been set in the first instance. If, however, the slow motion is used to make the final adjustment, damage is not likely to occur, as it lowers the body very gradually, and the latter is only pressed down upon the object by a spring. When using an oil-immersion lens a drop of cedar -wood oil should be placed on the object glass, and the body of the microscope racked down until the drop of oil touches the cover ; the final focussing can then be done with the fine adjustment. Some objects are so transparent that it is quite easy to pass by the focus and miss the correct position. In these cases dust on the cover glass maybe focussed first, and the fine adjustment lowered by an amount representing the thickness of the cover glass. If the slide be moved backwards and forwards on the stage during the process of focussing, the movement will be seen directly the correct position is nearly reached. It may appear absurd to mention that if a slide happens to have been placed on the stage upside down a high-power object glass will not focus through the thick glass slip, but the writer has more than once made such a mistake and wondered why he could not focus his specimen.
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Microscope Technique 
