FIG. 26.

brass socket let into the wooden support, thus completing the Microscope in the form represented in Fig. 25. This is not the last of the adaptations of which the instrument is capable; for the wooden support remaining at the same inclination, the body may be brought to the perpendicular, by shifting its stem in the clamp and by altering its angle at the cradle-joint; whilst a horizontal position may be given to the stage, by fitting it into another socket (Fig. 26); in this arrangement, moreover, the stage acquires an increase of firmness, from the bearing of a plate that projects at right angles from its under surface, upon the inclined face of the wooden support. Thus a dissecting microscope is formed, which has many of the advantages of that of Messrs. Smith and Beck; being subject, however, to the important drawback, that the mirror cannot be so placed as to reflect the light upwards through the axis of the microscope. (A means of remedying this, however, might perhaps be contrived without much difficulty or cost.) On the left side of the slanting support, at a short distance above the stage, is a hole into which may be fitted either the stem of a condensing lens for opaque objects, or the stem of the stage-forceps; either or both of which may also be fitted into holes in the front corners of the stage. The stage is provided with a sliding ledge for the support of objects in an inclined position; and it might also be furnished, if required, with a diaphragm plate. One of the chief merits of the instrument, however, being lightness and portability, it would not be desirable to encumber it with many accessories. For convenience of packing, the shorter portion of the piece may be connected with the longer by strong pins fitted into sockets, instead of being permanently fixed, so that the two can be readily disconnected and one part laid flat upon the other; and the whole apparatus will then lie within a very small compass. The distinctive peculiarity of this instrument consists in the extreme simplicity of the means by which a variety of useful ends are obtained. It is scarcely one that should be recommended to the beginner; since it is in several respects not so well adapted for ordinary work, as the forms already described. But it is a most valuable addition to the Microscopic apparatus of

Warington's Universal Microscope, arranged for dissecting on the stage.

the Naturalist; and may be constructed at so trifling an expense, to work with any objectives he already may possess, that a considerable demand may be anticipated for it.

37. We now pass to an entirely different class of instruments, -those of which the aim is, not simplicity but perfection; not the production of the best effect with limited means, but the attainment of everything that the Microscope can accomplish, without regard to cost or complexity. This object has been certainly carried out by the Opticians of our own country, much more completely than by those of the Continent; and it seems but fair towards the three principal London makers, by whose labors the present admirable results have been attained, that the pattern finally adopted by each should be here delineated and described. Without any invidious preference, the first place may fairly be

FIG. 27.

Ross's Large Compound Microscope.

are most liable to tremor; and we are that every part has been tested by the

assigned to the Large Compound Microscope of Mr. Ros8; not only as being the one which was first brought (in all essential features at least) to its present form, but also because it is that which contains the greatest number of provisions for investigating objects in a variety of different modes. The general plan of Mr. Ross's Microscope will be seen to be essentially the same with that which has been followed by Mr. Field in the simple form of this instrument first described (§ 31), as well as by many other makers; but it is carried out with the greatest attention to solidity of construction, in those parts especially which informed by Mr. Ross, "inverted pendulum"

'This instrument has been made for Mr. Warington and for the Author by Mr. Salmon, 100 Fenchurch Street: who supplies it, on either plan, without objectives or case, but with condenser and stage-forceps, for 3 guineas.

(an instrument contrived to indicate otherwise insensible vibrations), and either strengthened or reduced as might be found necessary, so as to obtain an equality of vibration between the stage and the optical part, which will prevent any perceptible tremor in the image. The "coarse" adjustment is made by the large milled head situated just behind the summit of the uprights, which turns a pinion working into a rack cut on the back of a very strong flattened stem, that carries the transverse arm at its summit; a second milled head (which is here concealed by the stage fittings) is attached to the other end of the axis of the pinion (as in Fig. 18), so as to be worked with the left hand. The "fine" adjustment is effected by the milled head on the transverse arm just behind the base of the "body;" this acts upon the "nose" or tube projecting below the arm, wherein the objectives are screwed. The other milled head seen at the summit of the stem, serves to secure the transverse arm to this, and may be tightened or slackened at pleasure, so as to regulate the traversing movement of the arm; this movement is only allowed to take place in one direction, namely, towards the right side, being checked in the opposite by a "stop," which secures the coincidence of the axis of the body with the centre of the stage and with the axis of the illuminating apparatus beneath it. It is in the movements of the stage, that the greatest contrivance is shown; these are three, namely, a traversing movement from side to side, a traversing movement from before backwards, and a rotatory movement. The traversing movements, which allow the platform carrying the object to be shifted about an inch in each direction, are effected by the two milled heads situated at the right of the stage; and these are placed side by side, in such a position that one may be conveniently acted on by the forefinger, and the other by the middle finger, the thumb being readily passed from one to the other. The traversing portion of the stage carries the platform whereon the object is laid, which has a ledge at the back for it to rest against; and this platform has a sliding movement of its own, from before backwards, by which the object is first brought near to the axis of the microscope, its perfect adjustment being then obtained by the traversing movement. To this platform, and to the traversing slides which carry it, a rotatory movement is imparted by a milled head, placed underneath the stage on the left hand side; for this milled head turns a pinion which works against the circular rack (seen in the figure) whereby the whole apparatus above is carried round about a third of a revolution, without in the least disturbing the place of the object, or removing it from the field of the microscope. This rotatory movement is useful for two purposes; first, in the examination of very delicate objects by oblique lights, in order that, without disturbing the illuminating apparatus, the effect of the light and shadow may be seen in every direction, whereby important additional information is often gained; and,

secondly, in the examination of objects under polarized light, a class of appearances being produced by the rotation of the object between the prisms, which is not developed by the rotation of either of the prisms themselves. Below the stage, and in front of the stem that carries the mirror, is a dovetail sliding bar, which is moved up and down by the milled head shown at its side; this sliding bar carries what is termed by Mr. Ross the "secondary stage" (omitted in the figure for the sake of simplicity), which consists of a cylindrical tube for the reception of the achromatic condenser, the polarizing prism, and other fittings; to this secondary stage, also, a rotatory motion is communicated by the turning of a milled head; and a traversing movement of limited extent is likewise given to it by means of two screws, one on the front and the other on the left hand side of the frame which carries it, in order that its axis may be brought into perfect coincidence with the axis of the "body." The special advantages of this instrument consist in its perfect steadiness, in the admirable finish of its workmanship, and in the variety of movements which may be given both to the object and to the fittings of the secondary stage. Its disadvantages consist in the want of portability, necessarily arising from the substantial mode of its construction; and in the multiplicity of its movable parts, which presents to the beginner an aspect of great complexity. complexity, however, is much more apparent than real; for each of these parts has an independent action of its own, the nature of which is very soon learned; and the various milled heads are so disposed, that the hand readily (and at last almost instinctively) finds its way from one to the other, so as to make any required adjustment, whilst the eye is steadily directed to the object. To the practised observer, therefore, this multiplication of adjustments is a real saving of time and labor, enabling him to do perfectly and readily what might otherwise require much trouble, besides affording him certain capabilities which he would not otherwise possess at all.

This

38. Powell and Lealand's Compound Microscope.-This instrument, represented in Fig. 28, is far lighter than the preceding in its general "build," without being at all deficient in steadiness; it has not, however, some of those improvements for which Mr. Ross's plan of construction is especially adapted. The three-legged stand gives a firm support to the trunnions that carry the tube to which the stage is attached, and from which a triangular stem is raised, by the rack-and-pinion movement set in action by the double milled head, whereby the "coarse" adjustment of the focus is obtained. The triangular stem carries at its summit the transverse arm, which contains (as in Mr. Ross's Microscope) the lever action of the "fine" adjustment; and this is acted on by the milled head at the back of the arm, whence also pass two oblique stays, which, being attached to the upper part of the body, assist in preventing its

FIG. 28.

vibration. The stage is provided with a traversing movement in each direction, to the extent of about three-quarters of an inch; this is effected on the plan known as Turrell's, in which the two milled heads are placed on the same axis, instead of side by side, one of them being also repeated on the left hand of the stage, so that the movements may be communicated either by the right hand alone, or by both hands in combination. The plat

form which carries the object is made to slide, as in the preceding case, on the summit of the traversing apparatus; and it has not only a ledge whereon the object may rest, but also a "spring clip" for securing it whenever the stage may be placed in a vertical position. This platform, moreover, is so connected with the traversing apparatus, that it may be turned round in the direction of its plane; but as

Powell and Lealand's Large Compound Microscope.

this rotation takes place above instead of beneath the traversing apparatus, there is no security that the centre of rotation shall coincide with the axis of the optical portion of the instrument; so that, unless this adjustment have been previously made, the object will be thrown completely out of the field of view when the platform is made to revolve. Hence, although this movement is of great use in facilitating the full examination of an object, by enabling the observer to bring it into the field of view in every variety of position, it does not serve, like the rotatory movement of Mr. Ross's stage, to change the position of the object in regard to the illuminating apparatus, without disturbing the observer's view of it. The condenser for transparent objects, the polarizing apparatus, &c., are here fitted to the under