MOVEMENTS AND FITTINnS OF THE SCOPE.
The microscope should be firmly planted on its support, so that there is no vibra tion when it is touched. There are two fundamental forms of support, the " horse shoe" seen in the Zeiss, Ross, and Leitz microscopes, and the tripod as in those of Swift, Watson, and Baker ; but, as a matter of fact, most makers now supply both forms in various patterns. A de sirable point is that when the microscope is placed in the horizontal position it should be quite as firmly fixed as when it is in the upright position. The tube of the microscope may be either 10 in. long (English model) or 6 in. long (Continental model), but the tendency at present is to construct instruments with a short outer tube and an inner graduated tube, which may be drawn out to make out ring, or a proper substage with rack and pinion motion and centring screws. Below the substage also is the mirror ; usually there are two mirrors, a plane one for parallel light, and a slightly concave one for condensing the light a little ; these are mounted back to back in the same ring, so that either may be used by simply reversing the ring.
Am' RO),I AT1C OBJECTIVES.
The most important portions of the op tical part of the microscope are the ob the combined length whatever may be de sired. Lengthening the tube in this way gives increased magnifying power without changing the lenses. When the objective alone is used for photomicrography, which is very often the case, it is convenient to unscrew the inner tube and connect the wide tube with the camera ; by so doing a large field is secured. The stage of the microscope may be a plain one, though a mechanical stage is preferable ; this is moved in two directions by milled heads at the side, so that after the glass slide has been placed in position it is not again touched with the fingers. Below the stage is a fitting for the condenser, polariscope, etc. ; this may be a fixed ring, a swing jectives ; they are designated as 2 in., 1 in., 2 in., 4 in., in., in., and so on, or by the Continental measures, 24, 16, 12, 8, 6, 4 mm., etc. These numbers repre sent, not the focal distances of the lenses, but the focal distances of simple lenses having equivalent magnifying powers. Objectives are now always constructed in such a way that they are achromatic ; that is, they consist of a combination of lenses of flint glass and of crown glass, the rays of light refracted by the latter being used, when an eyepiece giving a very high amplification, or when other than chromatic light is used, then these jectives " break down " and show colour fringes round the object or on the limit of the field. This is due to the fact that
only two colours of the spectrum are united in one point, and there is a residual or a so-called " secondary " spectrum i? corrected for. This defect, ho.wever, is united by the former, and brought into focus at the same point. Thus the light which passes through them is practically colourless ; but under certain conditions, for instance, when very oblique light is not noticeable under ordinary conditions, as the objectives now made are infinitely better in this respect than the older forms. The improvements that have been made in microscopic objectives are largely clue to the firm of Schott, of Jena, who have introduced several new kinds of glass known as borate, phosphate, and baryta glasses, and to the late Professor Abbe and other eminent physicists who have made it possible to calculate the form of lenses best suited for any particular pur pose.
.ArOCHROMATIC OBJECTIVES.
Carl Zeiss, of Jena, has produced a series of objectives which may be re garded as embodying an entirely different principle to those previously in use ; these objectives, termed by Professor Abbe " apo chromatic " objectives, are so constructed as to unite three different colours of the spectrum in one point of the axis; thus the so-called " secondary " spectrum is elim inated, and whether they are used with monochromatic light, artificial light, or daylight, the results are equally good, the images produced by all the colours of the spectrum being nearly equally sharp. These lenses, however, suffer from the same defect as all objectives of high aperture—that is, certain colour defects are visible in the extra axial portion of •the visual field (chromatic difference of magnification) ; the image formed by the blue rays being larger than that formed by the red, colour fringes are thus ob served increasing towards the limit of the field.