is so turned that the side represented as undermost in the figure, shall be uppermost; and it is in order that high powers may be used in this case as in the other, that the disk which then covers the object is made of thin glass, instead of being (as in the original form of the instrument) a piece of thick glass plate. That a thin glass disk is more liable to fracture under pressure, than a thick one, is no serious objection to its use for this purpose; since the lower one is not more likely to break than the upper one; and either may be replaced with extreme facility, by simply warming the part of the instrument to which it is attached, so as to loosen the cement that holds it. And the advantage of being able to view an object under a high power, from either side, will be most fully appreciated by

FIG. 51.

A B C

every one who has been much engaged in the class of observations, which this instrument is specially adapted to facilitate. If this Compres sorium be made of sufficient size to admit an ordinary glass slide between the vertical pins, an object may be subjected to compression, and afterwards removed for examination out of the compressor, without transferring it from one glass to another, which is frequently an advantage. In this case, it will be convenient that the thin glass disk should be "countersunk" into the upper surface of the principal plate, so as to form one level with it.

71. Dipping Tubes.-In every operation in which small quantities of liquid, or small objects contained in liquid, have to be dealt with by the Microscopist, he will find it a very great convenience to be provided with a set of tubes of the forms represented in Fig. 51, but of somewhat larger dimensions. These were formerly designated as "fishing tubes;" the purpose for which they were originally devised having been the fishing out of Water-Fleas, aquatic Insect-Larvæ, the larger Animalcules, or other living objects distinguishable either by the unaided eye or by the assistance of a magnifying glass, from the vessels that may contain them. But they are equally applicable, of course, to the selection of minute Plants; and they may be turned to many other no less useful purposes, some of which will be specified hereafter. When it is desired to secure an object which can be seen either with the eye alone or with a magnifying glass, one of these tubes is passed down into the liquid, its upper orifice having been previously closed by the forefinger, until its lower orifice is close above the object; the finger being then removed, the liquid suddenly rises into the tube, probably carrying

Fishing Tubes.

the object up with it; and, if this is seen to be the case, by putting the finger again on the top of the tube, its contents remain in it when the tube is lifted out, and may be deposited on a slip of glass or on the lower disk of the aquatic box, or, if too copious for either receptacle, may be discharged into a watch-glass. In thus fishing for any but the minutest objects, it will be generally found convenient to employ the open-mouthed tube c; and when its contents have been discharged, if they include but a single object of the desiderated kind, this may be taken up by one of the finer tubes, A, B, or, if more convenient, the whole superfluous fluid may be sucked up by the mouth, and the object left with no more than is suitable; or, if there be many of the desired objects in the fluid first selected, these may be taken up from it, one by one, by either of the finer tubes. 72. Forceps. Another instrument so indispensable to the Microscopist as to be commonly considered an appendage to the Microscope, is the Forceps for taking up minute objects; many forms of this have been devised, of which one of the most convenient is represented in Fig. 52, of something less than the

FIG. 52.

Forceps.

actual size. As the forceps, in marine researches, have continually to be plunged into sea-water, it is better that they should be made of brass or of German silver, than of steel, since the latter rusts far more readily; and as they are not intended (like dissecting forceps) to take a firm grasp of the object, but merely to hold it, they may be made very light, and their spring part slender. As it is essential, however, to their utility, that their points should meet accurately, it is well that one of the blades should be furnished with a guide-pin, passing through a hole in

the other.

The foregoing constitute, it is believed, all the most important pieces of Apparatus, which can be considered in the light of Accessories to the Microscope. Those which have been contrived to afford facilities for the preparation and mounting of Objects, will be described in a future chapter (Chap. V). It may be thought that some notice ought to be taken of the Frog Plate and Fish Pan, with the former of which many Microscopes are supplied, whilst the latter has scarcely yet gone altogether out of use. But the Author having been accustomed to gain all the advantages of these, by methods far more simple, whilst at least equally efficacious, does not consider them as presenting any advantages which render it desirable to expend time or space in giving a detailed account of them; and he will explain the methods alluded to, under the appropriate head.

CHAPTER IV.

MANAGEMENT OF THE MICROSCOPE.

73. Support.-The Table on which the Microscope is placed, when in use, should be one whose size enables it also to receive the various appurtenances which the observer finds it convenient to have within his reach, and whose steadiness is such as to allow of his arms being rested upon it without any yielding; it should, moreover, be so framed, as to be as free as possible from any tendency to transmit the vibrations of the building or floor whereon it stands.' The manner in which the Microscope itself is constructed, however, will have a great influence on the effect of any such disturbing cause; since, if the whole instrument move together, scarcely any tremulousness will be produced in the image, by vibrations which cause it to "dance" most unpleasantly, if the body and stage of the Microscope oscillate independently of each other. Hence, in choosing a Microscope, it should always be subjected to this test, and should be unhesitatingly rejected if the result be unfavorable. It is of course to be borne in mind, that any vibration, either of the object or of the optical apparatus, in which the other does not partake, will be much more apparent when high magnifying powers are used, than when the object is amplified in a much less degree, the motion of the object being magnified in precisely the same ratio with the object itself; hence if, when the microscope is thus tested with high powers, it is found to be free from fault, its steadiness with low powers may be assumed; but, on the other hand, a Microscope which may give an image free from perceptible tremor when the lowest powers only are employed, may be quite unfit for use with the highest.

74. Light. Whatever may be the purposes to which the Microscope is applied, it is a matter of the first importance to secure a pure and adequate illumination. There is scarcely any class

1 The working Microscopist will find it a matter of great convenience to have a Table specially set apart for this purpose; furnished with drawers in which are contained the various accessories he may require for the preparation and mounting of objects. If the Microscope be one which is not very readily taken out from and put back into its case, it is very convenient to cover it with a large bell-glass; which may be so suspended from the ceiling, by a cord carrying a counterpoise at its other end, as to be raised or lowered with the least possible trouble, and to be entirely out of the way when the Microscope is in use. Similar but smaller bell-glasses are also useful for the protection of objects, which are in course of being examined or prepared, and which it is desirable to seclude from dust.

of objects, for the examination of which good daylight is not to be preferred to any other kind of light; but good lamplight is preferable to bad daylight. When daylight is employed, the Microscope should be placed near a window, whose aspect should be (as nearly as may be convenient) opposite to the side on which the sun is shining; for the light of the sun reflected from a bright cloud, is that which the experienced Microscopist will almost always prefer, the rays proceeding from a cloudless blue sky being by no means so well fitted for his purpose, and the dull lurid reflection of a dark cloud being the worst of all. The direct rays of the sun are far too powerful to be used with advantage, unless its intensity be moderated, either by reflection from a plaster-of-paris or some other "white-cloud" mirror (§ 58), or by passage through some imperfectly transparent medium. The moderator contrived by Mr. Rainey for lamp or gas-light ($75), has been found to answer equally well for direct sunlight; the glare and heating power of which it so effectually subdues, as to destroy all tendency to injure the most delicate object, or to confuse the observer's view of it; whilst an illumination is obtained by its means, whose intensity renders it superior for certain purposes to anything else. The young Microscopist is earnestly recommended to make as much use of daylight as possible; not only because, in a large number of cases, the view of the object which it affords is more satisfactory than that which can be obtained by any kind of lamp-light, but also because it is much less "trying" to the eyes. So great, indeed, is the dif ference between the two in this respect, that there are many who find themselves unable to carry on their observations for any length of time by lamp-light, although they experience neither fatigue nor strain from many hours' continuous work by daylight.

75. When recourse is had to Artificial light, it is of great importance, not only that it should be of good quality, but that the arrangement for furnishing it should be suitable to the special wants of the Microscopist. Thus, although a wax or composition candle affords a very pure light, yet its use is attended with two inconveniences, which render its use very undesirable when any better light can be obtained ;-namely, the constant flickering of the flame, which is not sufficiently prevented by surrounding it with a chimney; and the continual alteration in its level, which is occasioned by the consumption of the candle. The most useful light for ordinary use, is that furnished by the steady and constant flame of the lamp, fed either with oil, camphine, or gas; the wick or burner should be cylindrical or "argand;" it should be capable of adjustment to any height above the table; and a movable shade should be provided, by which the light may be prevented from coming direct to the observer's eyes, or from diffusing itself too widely through the room. These requisites are supplied by the lamp commonly known as the "University"

or "reading" lamp, which has a circular foot with a vertical stem, on which the oil-reservoir (carrying with it the burner) and the shade, can be fixed at any convenient height. French and German lamps on the same general construction, but having the reservoir contrived on the "bird-fountain" principle, are also to be obtained, being largely imported for the use of watch-makers; these have the advantage of burning out all their oil, which is not the case with the ordinary "reading"-lamp, as it does not burn well except when full or nearly so; but they are usually destitute of a shade, which, however, can be easily added. Lamps of either kind are sometimes constructed on the "solar" principle, which increases the purity and intensity of the light, but at the same time not only diminishes the diameter of the flame, but also produces an inconvenient transverse "break” near its lower part. The best kind of light which an oil-lamp can furnish, is that yielded by the "Moderator" lamps which have of late come into such general use; but they have this important drawback, that they contain in themselves no adjustment for varying the elevation of the burner, and that their construction is such as to give no facilities for any arrangement of this kind. The same objection applies to the Camphine-lamps in ordinary use; but a small camphine-lamp has been constructed for the special use of Microscopists, which is capable of being placed on an adjustable stand, so that its flame may be raised or lowered to any desired level. The light of this lamp is whiter and more intense than that of any other, and it may be used with advantage for certain very delicate observations (§ 58); but for the ordinary purposes of the Microscopist it is not so convenient, the surface of flame from which the light can be received by the mirror or condenser, being limited by the peculiar construction which the combustion of camphine requires. To every one who has a supply of gas at command, the use of it for his microscopelamp (by means of a flexible tube) strongly recommends itself, on account of its extreme convenience, and its freedom from any kind of trouble. The lamp should be constructed on the general plan already described, the burner being made to slide up and down on a stem rising perpendicularly from a foot, which also carries a shade; and the burner should be one which affords a bright and steady cylindrical flame, either "Leslie's" or the "cone"-burner being probably the best. Even the best light supplied by a gas-lamp, however, is inferior in quality to that of a good oil-lamp; and is more injurious and unpleasant to the eye. Hence the interposition of some kind of artificial medium, adapted to keep back the yellow rays, whose predominance in the lamp-flame is the chief source of its injurious action, is especially required when gas-light is used. This may be partly effected, by the simple expedient of using a chimney of bluish glass, known as "Leblond's;" but, in addition, it is advantageous to cause the light to pass through a screen of bluish-black or