(F) MECHANICAL CONSTRUCTION OF THE MICROSCOPE

The optical arrangements on which the, working of the compound achromatic microscope depends having now been explained, we have next to consider the mechanical provisions whereby they are brought to bear upon the different purposes which the instrument is destined to serve. Every complete microscope must possess, in addition to the lens or combination of lenses which affords its magnifying power, a stage whereon the object may securely rest, a concave mirror for the illumination of transparent objects from beneath, and a condensing-lens for the illumination of opaque objects from above.

1. Now, in whatever mode these may be connected with each other, it is essential that the optical part and the stage should be so disposed as either to be altogether free from tendency to vibration or to vibrate together; since it is obvious that any movement of one, in which the other does not partake, will be augmented to the eye of the observer in proportion to the magnifying power employed. In a badly-constructed instrument, even though placed upon a steady table resting upon the firm floor of a well-built house, when high powers are used, the object is seen to oscillate so rapidly at the slightest tremor—such as that caused by a person walking across the room, or by a carriage rolling by in the street frequently almost indistinguishable; whereas in a well-constructed instrument scarcely any perceptible effect will be produced by even greater disturbances. Hence, in the choice of a microscope it should always be subjected to this test, and should be unhesitatingly rejected if the result be unfavourable. If the instrument should be found free from fault when thus tested with high powers, its steadiness with low powers may be assumed; but, on the other hand, though a microscope may give an image free from perceptible tremor when the lower powers only are employed, it may be quite unfit for use with the higher. The method still adopted by some makers, of supporting the body by its base alone, is the worst possible, especially for the long body of the large English model, since any vibration of its lower part is exaggerated at its ocular end. The firmer the support of the body along its length the less tremor will be seen in the microscopic image.

2. The next requisite is a capability of accurate adjustment to every variety of focal distance, without movement of the object. It is a principle universally recognized in the construction of good microscopes that the stage whereon the object is placed should be a fixture, the movement which the focus is to be adjusted being given to the optical portion. This movement should be such as to allow free range from a minute fraction of an inch to three or four inches, with equal power of obtaining a delicate adjustment at any part. It should also be so accurate that the optic axis of the instrument should not be in the least altered by any movement in a vertical direction, so that, if an object be brought into the centre of the field with a low power, and a higher power be then substituted, the object should be found in the centre of its field, notwithstanding the great alteration in the focus. In this way much time may often be saved by employing a low power as a "finder" for an object to be examined by a higher one; and when an object is being viewed by a succession of powers little or no readjustment of its place on the stage should be required. A rack-and--pinion adjustment, if it be made to work both tightly and smoothly, answers sufficiently well for the focal adjustment, when objectives of low power only are employed. But for any lenses whose focus is less than half an inch a "fine adjustment," or "slow motion," by means of a screw-movement operating either on the object-glass alone or on the entire body (preferably on the latter), is of great value; and for the highest powers it is quite indispensable. It is essential that in this motion there should e no "lost time," and that its working should not produce any "twist" or displacement of the image. In some microscopes which are provided with a fine adjustment the rack-and-pinion movement is dispensed with, the "coarse adjustment" being given by merely sliding the body up and down in the socket which grasps it; but this plan is only admissible where, for the sake of extreme cheapness or portability, the instrument has to be reduced to the form of utmost simplicity, as in figs. 28, 29.

3. Scarcely less important than the preceding requisite, in the case of the compound microscope, especially with the long body of the ordinary English model, is the capability of being placed in either a vertical or a horizontal position, or at any angle with the horizon, without deranging the adjustment of its parts to each other, and without placing the eye-piece in such a position as to be inconvenient to the observer. It is certainly a matter of surprise that. some microscopists, especially on the Continent, should still forego the advantages of the inclined position, these being attainable by a very small addition to the cost of the instrument; but the inconvenience of the vertical arrangement is much less when the body of the microscope is short, as in the ordinary Continental model and there are many cases in which it is absolutely necessary that the stage should be horizontal. This position, however, can at any time be given to the stage of the inclining microscope, by bringing the optic axis of the instrument into the vertical direction. In ordinary cases, an inclination of the body at an angle of about 55º to the horizon will usually be found most convenient for unconstrained observation; and the instrument should be so constructed as, when thus inclined, to give to the stage such an elevation above. the table that, when the hands are employed at it, the arms may rest conveniently upon the table. In this manner a degree of support is attained which gives such free play to the muscles of the bands that movements of the greatest nicety may be executed by them, and the fatigue of long-continued observation is greatly diminished. When the ordinary camera lucida [Footnote 275-1] is used for drawing, or measuring, it is requisite that the microscope should be placed horizontally. It ought, therefore, to be made capable of every such variety of position; and the stage must of course be provided with some means of holding the object, whenever it is itself placed in such a position that the object would slip down unless sustained.

4. The last principle on which we shall here dwell, as essential to the value of a microscope designed for ordinary work, is simplicity in the construction and adjustment of every part. Many ingenious mechanical devices have been invented and executed for the purpose of overcoming difficulties which are in themselves really trivial. A moderate amount of dexterity in the use of the hands is sufficient to render most of these superfluous; and without such dexterity no one, even with the most complete mechanical facilities, will ever become a good microscopist. There is, of course, a limit. to this simplification; and no arrangement can be objected to on this score which gives advantages in the examination of difficult objects, or in the determination of doubtful questions, such as no simpler means can afford. The meaning of this distinction will become apparent if it be applied to the cases of the mechanical stage and the achromatic condenser. For, although the mechanical stage may be considered a valuable aid in observation, as facilitating the finding of a minute object, or the examination of the entire surface of a large one, yet it adds nothing to the clearness of our view of either; and its place may in great degree be supplied by the fingers of a good manipulator. On the other hand, the use of the achromatic condenser not only contributes very materially, but is absolutely indispensable, to the formation of a perfect image, in the case of many objects of a difficult class; the want of it cannot be, compensated by the most dexterous use of the ordinary appliances; and consequently, although it may fairly be considered superfluous as regards a large proportion of the purposes to which the microscope is directed, whether for investigation or for display, yet as, regards the particular objects just alluded to it is a no less necessary part of the instrument than the achromatic objective itself.





As a typical example of the simplest form of compound microscope that is suitable for scientific research,—which, with various modifications of detail, is the one generally employed on the Continent,—the Microscope de dissection et d’observation (fig. 28) of M. Nachet, especially as constructed for portability (figs. 29-31), seems particularly worthy of description. In its vertical form (fig. 28) the solid foot to which the mirror is pivoted gives support to the pillar F, to the top of which the stage P, having a diaphragm-plate beneath it, is firmly attached. On the top of this pillar the tubular stem A is fitted in such a manner that it may be removed by unscrewing the large milled head L,—though, when this is well screwed down, the stem stands quite firmly. This stem bears at its summit a short horizontal arm, which carries a strong vertical tube that firmly grasps the "body" of the microscope, while permitting this to be easily slid upwards or downwards, so as to make a "coarse adjustment" of the focus. The "fine adjustment" is made by turning the milled bead V, which either presses down the, outer tube of the stem, or allows it to be raised by the upward pressure of a strong spiral spring in its interior. By unscrewing the milled head L, the stem A with its arm and compound body can be detached from the pillar; and, a small light arm H holding either single lenses or doublets being slid into this, a convenient dissecting microscope is thus provided. The only drawback in the construction of this simple model is its not being provided with a joint for the inclination of the body; but this is introduced into the portable form of the instrument shown in fig. 29, the basal portion of which (fig. 30) can be used, like that of the preceding model, as a simple microscope, and, by a most ingenious construction, can be so folded as to lie, flat in a shallow case (fig. 31) that holds also the upper part with the objectives of both the simple arm and the compound body. M. Nachet now connects his objectives with the body of his microscopes, not by a screw, but by a cylindrical fitting held in place by the pressure of a spring-clip against a projecting shoulder. This method not only allows one objective to be removed and replaced by another much more readily than does the screw-fitting, but also renders the centring of different objectives more exactly coil formable. It may be safely affirmed that a very large proportion of the microscope work of the last half- century, which has given an entirely new aspect to biological science, has been done by in-struments of this simple Continental type.

A larger model, however, was from the first adopted by English opti-cians; and, as a typical example of the general plan of construction now most followed both in England and in the United States, the improved Jackson- Zentmayer micro-scope of Messrs Ross (fig. 32) may be appropriately selected. The tripod base of this instrument carries two pillars, between which is swung upon a horizontal axis (capable of being fixed in any position by a tightening screw) a solid "limb," with which all the other parts of the instrument are connected,—a plan of construction originally devised by Mr George Jackson. The binocular body, having at its lower end (as in fig. 24) an opening into which either of the Wenham prisms can be inserted, and at its top a rack movement for adjusting the eye pieces to the distance between the eyes of the observer, is attached to a racked slide, which is so acted on by the large double milled head in the upper part of the limb as to give a "quick" upward or downward motion to the body; while the "slow" motion, or fine adjustment, is given by means of the vertical micrometer screw at the back of the limb, which raises or lowers a second slide behind the rack. [Footnote 276-1] The stage is supported upon a firm ring, which is immovably fixed, not to the limb, but to a strong conical pivot which passes through the limb, to be clamped by a screw-nut at its back—the purpose of this being to allow the whole stage to be inclined to one side or the other at any angle, so that a solid object may be viewed sideways or from below, as well as from above. Upon this ring the stage rotates horizontally, its angular movement being measured by a graduated scale and vernier at its edge; and it can be fixed in any azimuth by a clamping-crew beneath. Rect-angular movement is given to the traversing platform which carries the object by two milled heads on the right of the stage, the whole construction of which is adapted to allow light of extreme obliquity to be thrown upon the object from beneath. On the strong pivot by which the stage is attached to the limb (the axis of which passes through the point at which the object-plane is intersected by the optic axis of the body) is hung the swinging tail-piece invented by Mr Zentmayer of Philadelphia, which, carrying the whole illuminating apparatus, may be so set as to give to the axis of the illuminating pencil any required degree of obliquity. To the upper part of it is attached a rack-and-pinion movement carrying the "substage," which is provided with two milled-headed screws for centring it precisely with the microscope-body. Into this may be fitted the achromatic condenser, parabolic illuminator, polarizing prism, or any other kind of illuminating apparatus; whilst at its lower end it carries the mirror, the position of which may be varied by sliding its fitting up or down the "tail-piece," or by turning the arm which carries it to one side or the other; while, if direct illumination from a lamp should be preferred, it may be turned altogether aside. By swinging the tail-piece round above the stage, oblique light may be reflected from the mirror, through the condenser, upon the upper surfaces of objects. The condenser usually fitted to this instrument is of about 4/10 inch focus, with a large back lens; behind which are placed an iris-diaphragm for reducing the light to the central rays, and a diaphragm-plate with apertures of the various forms most suited for the resolution of lined objects by oblique rays.

No instrument, in the writer’s judgment, is better adapted than this for the highest purposes of microscopical research. It works admirably with every power from the lowest to the highest, and is capable of receiving any one of the numerous pieces of apparatus which have been devised for special researches of various kinds. The detailed description of these not being here admissible, it will be sufficient to indicate the polariscope and the spectroscope as the most important of these accessories.





Footnotes

275-1 A camera lucida adapted for use with the vertical microscope has been devised by M. Nachet.

276-1 In the older form of construction still retained by some makers the fine adjustment acts directly on the objective, the fitting of which is made to slide up and down within the nose of the body; but this plan is attended with many disadvantages.


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