prism ABC. By internal reflexion from the former and external reflexion from the latter about half the beam b is reflected within the first prism in the direction cb, while the other half proceeds straight onwards through the second prism, in the direction ca , so as to pass into the principal body. The reflected half, meeting at d the oblique (silvered) surface DE of the first prism, is again reflected in the direction db , and, passing out of that prism per pendicularly to its surface AF, proceeds towards the secondary body. The two prisms must not be in absolute contact along the plane AB, since, if they were, Newton s rings would be formed ; and much nicety is required in their adjustment, so that the two reflexions may be combined without any blurring of the image in the secondary body. For the prolonged observation, under high powers, of objects not requiring the extreme of perfection in definition, such, for example, as the study of the cyclosis in plants, great advantage is gained from the conjoint use of both eyes by one of the above arrangements. 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 coudeusing- 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 vibra tion 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 as to be 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 by 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 in strument 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, notwithstand ing 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-move ment 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 be no "lost time," and that its working should not produce any "twist " or displacement of the image. In some micro scopes 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. 275 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 incon venient 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 incon venience 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 55 to the horizon will usually be found most convenient for uncon strained 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 hands 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 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 simpli city in the construction and adjustment of every part. Many in genious 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 micro scope 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 micro scope that is suitable for scientific research, which, with various modifications of detail, is the one generally employed on the Con tinent, the Microscope de dissection et d observation (fig. 28) of 11. 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 tubu lar 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 per mitting 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 head V, which either presses down the outer tube of the stem, or allows it to be raised by the upward pres sure 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 II holding either single lenses or doublets being slid into this, a convenient dissecting microscope is thus provided. The only drawback in the construc tion of this simple model is its not being provided with a joint for i A camera lucida adapted for use with the vertical microscope has been de
vised by M. Naclirt.