Almost any make of high-grade instrument will give satisfaction that has the following qualities:
1. It should possess firmness and solidity through its base, pillar, arm, etc.
2. It should have a good-sized stage, preferably square, thick, firm, of glass, metal, or vulcauite.
3. It should have coarse and fine adjustments, worked by rack and pinion; both should move evenly, smoothly, promptly, without wobbling.
4. It should possess working distance; thus the higher objective when in focus should leave sufficient space above the stage for the introduction of slides, etc., without danger of contact. The longer the working distance, the less will the distinctness of the image formed be affected by any given alteration in its focal adjustment. Lenses with greatest working distance have most focal depth.
5. A nose-piece is essential in order to economize time and facilitate work. This may be either double, triple, or quadruple, and is an appliance fitting the tube's extremity for carrying 2, 3, or 4 objectives, of varying power, any one of which may quickly be brought into direct position by turning the arm on a pivot.
6. The penetrating power (focal depth or range of focus) should be considerable. This quality, though not necessary for very thin sections, enables one to see the parts of an object not exactly in focus with sufficient distinctness to allow their relations with what lies exactly in that plane to be clearly traced out. Thus one lens may only for an instant give a sharp focus at a limited distance from the object, while another lens may give a good image at a considerable distance above and below the best focal point; the first kind of lens prevents us from ascertaining the relation of the higher layers of an object to the lower unless we continually follow the focus with the fine adjustment; the second kind of lens, having greater penetration, brings a thicker portion of the object into view at the same time - the greater the penetrating power, the better the microscope.
7. Flatness of the field varies with the magnifying power and angle of aperture of the lens. Here all parts of the field are in focus at the same time, so that the image is distinct over the whole field at once without marginal color. This requisite should be tested for under an eye-piece giving a large aperture.
8. The distinctness of the image (defining power) should be good;
this depends upon the complete correction of chromatic and spherical aberrations, and upon the accurate centering of the lenses, otherwise the outer borders will be blurred.
9. Resolving power, by which very minute and closely approximated markings, lines, striae, dots, and apertures can be discerned separately; the maximum capacity thus far attained being the separation of 118,000 lines per linear inch. These three last qualities are very essential.