Mechanical Portion Microscope

Written by microscopes

Saturday, 15 September 2007

Information about the mechanical portion of the microscope.

Mechanical Portion of the Microscope

In directing attention to the mechanical portion of the microscope, I must say a few words upon the arrangements for altering the focus, the body of the instrument, the diaphragm, and the stage.

Adjustments for altering the Focus:

The ordinary movement is obtained by the rack and pinion. In some microscopes the body is moved by the fingers alone, and is arranged to slide in a tube (which may be lined with cloth) like a telescope. Besides coarse adjustment, however, every microscope should be provided with a more delicate movement for altering the focus when high powers are employed. The details of the arrangement of the fine adjustment are different in various instruments.

The Body of the Microscope:

The instrument should be perfectly steady, whether the body be inclined or arranged in a vertical position; and not the slightest lateral movement or vibration should be communicated to the body of the microscope when the focus is altered by turning either of the adjustment screws. The base or foot should be sufficiently heavy to give steadiness, and should touch the ground in three places only, or the body should be fixed upon three feet. The foot or base may be made of cast iron, or even of zinc, or some other cheap metal.

The body ought to be provided with a joint, so that it may be inclined or placed in a horizontal position, as is requisite when drawings are made with the camera, or when objects are measured by the aid of that instrument or the steel disc or neutral tint glass reflector. Another advantage gained by this moveable joint is that the muscles of the observer's neck do not become so tired when the body of the microscope is inclined as when the head has to be bent, for several hours at a time, over an instrument standing upright. The larger the microscope may be, the more necessary is this joint for the comfort of the observer; and as it in no way impairs the steadiness of the instrument, and only adds a few shillings to the expense, I recommend every one, in choosing a microscope, to select an instrument the body of which may be placed in a vertical, inclined, and horizontal position.

The Stage:

The stage should be at least three inches in length by two and a half in width, and there should be a distance of at least an inch and a half from the centre of the opening in the stage over which the slide is placed, to the upright pillari. I. The stages of the microscopes of Nachet, Oberhauser, and indeed those of most of the foreign makers are too contracted for convenience. A good large stage is a great advantage, and it should be so arranged that a small saucer can be placed upon it and moved freely in various directions. A piece of thin plate-glass, of a required size, may be made to fit on the stage, and thus a small stage is easily converted into a large one.

Diaphragm:

Beneath the stage a circular diaphragm plate with holes in it of several different sizes, should be so arranged that it can be made to revolve without difficulty and any hole brought under the object; a catch is of great advantage, as it tells the observer when each particular opening reaches the centre of the field. Various arrangements have been adopted for altering the size of the aperture in the diaphragm instead of having a revolving plate with several holes of different sizes. One of the most ingenious is that devised by Mr. B. Kincaid (Mic. Journal, July, 1866, p. 75). This is made of a short piece of thin India-rubber tube, the two ends of which, fixed to brass rings, are made to revolve in opposite direction, so that the central part becomes contracted. An aperture of any size may be obtained, and the opening must be always perfectly central.