production of Stereoscopic effects will be produced; but for certain classes of objects, this mode of exhibition is most admirably adapted, the solid forms of the Polycystina (Chap. X), for example, being brought out by it (especially when they are viewed as opaque, not as transparent objects) with such a reality, as to make them resemble carved ivory balls which the hand feels ready to grasp.

41. The same method of dividing the pencil of rays issuing from the object glass, by a separating prism placed in its course, has been applied by M. Nachet to another purpose, that of enabling two or more observers to look at the same object at once, which is often a matter not only of considerable convenience, but also of great importance, especially in the demonstration of dissections. The account given by M. Nachet of the construction of this instrument, as adapted for two persons, will be found in the "Quarterly Journal of Microscopical Science," Vol. II, p. 72; he has subsequently devised another arrangement, by which the form of the separating prism is adapted to divide the pencil into three or even four parts, each of which may be directed into a different body, so as to give to several observers at one time a nearly identical image of the same object. Of course, the larger the number of secondary pencils into which the primary pencil is thus divided, the smaller will be the share of light which each observer will receive; but this reduction does not interfere with the distinctness of the image, and may be in some degree compensated by a greater intensity of illumination. (See Appendix for a description of American instruments and modifications.)

CHAPTER III.

ACCESSORY APPARATUS.

42. IN describing the various pieces of accessory apparatus with which the Microscope may be furnished, it will be convenient in the first place to treat of those which form (when in use) part of the instrument itself, being Appendages either to its Body or to its Stage, or serving for the Illumination of the objects which are under examination; and secondly to notice such as have for their function to facilitate that examination, by enabling the microscopist to bring the Objects conveniently under his inspection.

SECTION 1. APPENDAGES TO THE MICROSCOPE.

43. Draw-Tube.-It is advantageous for many purposes, that the Eye-piece should be fitted, not at once into the "body" of the Microscope, but into an intermediate tube; the drawing out of which, by augmenting the distance between the object-glass and the image which it forms in the focus of the eye-glass, still further augments the size of the image in relation to that of the object (§ 20). For although the magnifying power cannot be thus increased with advantage to any considerable extent, yet, if the corrections of the object-glass have been perfectly adjusted, its performance is not seriously impaired by a moderate lengthening of the body; and this may be conveniently had recourse to on many occasions, in which some amplification is desired, intermediate between the powers furnished by any two objectives. Thus if one object-glass give a power of 80 diameters, and another a power of 120, by using the first and drawing the eyepiece, its power may be increased to 100. Again, it is often very useful to make the object fill up the whole, or nearly the whole, of the field of view: thus if an object that is being viewed by transmitted rays, is so far from transparent as to require a strong light to render its details visible, the distinctness of its details is very much interfered with, if, through its not occupying the peripheral part of the field, a glare of light enter the eye around its margin; and the importance of this adjustment is even greater, if opaque objects mounted on black disks are being viewed by

the Lieberkühn (§ 65), since if any light be transmitted to the eye direct from the mirror, in consequence of the disk failing to occupy the centre field, it greatly interferes with the vividness. and distinctness of the image of the object. In the use of the Micrometric eye-pieces to be presently described (§§ 45, 46), very great advantage is to be derived from the assistance of the drawtube; as enabling us to make a precise adjustment between the divisions of the stage micrometer, and those of the eye-piece micrometer; and as admitting the establishment of a more convenient numerical relation between the two, than could be otherwise secured without far more elaborate contrivances. Moreover, if, for the sake of saving room in packing, it be desired to reduce the length of the body, the draw-tube affords a ready means of doing so; since the body may be made to "shut up,' like a telescope, to little more than half its length, without any impairment of the optical performance of the instrument when mounted for use.

FIG. 32.

44. Erector. It is only, however, in the use of the Erector, that the full value of the draw-tube, and the advantage of giving to it a rack-and-pinion movement of its own (§ 35), come to be fully appreciated. This instrument, first applied to the Compound Microscope by Mr. Lister, consists of a tube about three inches long, having a meniscus at one end and a plano-convex lens at the other (the convex sides being upwards in each case), with a diaphragm nearly half way between them; and this is screwed into the lower end of the draw-tube, as shown in Fig. 32. Its effect is (like the corresponding erector of the Telescope), to antagonize the reversion of the image formed by the object-glass, by producing a second reversion, so as to make the image presented to the eye correspond in position with the object. The passage of the rays through two additional lenses, of course occasions a certain loss of light by reflection from their surfaces, besides subjecting them to aberrations whereby the distinctness of the image is somewhat impaired; but this need not be an obstacle to its use for the class of purposes for which it is especially adapted in other respects (§ 35), since these seldom require a Draw-tube fitted very high degree of defining power. By the position given to the Erector, it is made subservient to another purpose, of great utility; namely, the procuring a very extensive range of magnifying power, without any change in the objective. For when the draw-tube, with the erector fitted to it, is completely pushed in, the acting length of the body (so to speak) is so greatly reduced by the formation of the first image much nearer the objective, that, if a lens of 8-10ths of an inch focus be employed, an object of the diameter of 13 inch can be taken in,

with Erector.

and enlarged to no more than 4 diameters; whilst, on the other hand, when the tube is drawn out to its whole length, the object is enlarged 100 diameters. Of course every intermediate range can be obtained, by drawing out the tube more or less; and the facility with which this can be accomplished, renders such an instrument most useful in various kinds of research, especially those in which it is important, after finding an object with a lower power, to examine it under a higher amplification; since this may be done, without either a change of objectives, or a transfer of the object to another microscope fitted with a different power. It is when the draw-tube is thus made subservient to the use of the Erector, that the value of its rack-and-pinion adjustment is most felt; for by giving motion to the milled head which acts upon this (Fig. 22) with one hand, whilst the other hand is kept upon the milled head which moves the whole body (it being necessary to shorten the distance between the object and the objective, in proportion as the distance of the image from the objective is increased), the observer-after a little practice in the working together of the two adjustments-may almost instantaneously alter his power to any amount of amplification which he may find the object to require, without ever losing a tolerably distinct view of it. This can scarcely be accomplished without the rack movement; since, if both hands be required to make the alteration of the draw-tube, the readjustment of the focus must be effected subsequently.

45. Micrometer.-Although some have applied their micrometric apparatus to the stage of the microscope, yet it is to the Eye-piece that it may be most advantageously adapted. The cobweb micrometer, invented by Ramsden for Telescopes, is probably, when well constructed, the most perfect instrument that the Microscopist can employ. It is made by stretching across the field of a "positive" eye-piece (§ 23) two very delicate parallel wires or cobwebs, one of which can be separated from the other by the action of a fine-threaded screw, the head of which is divided at its edge into a convenient number of parts, which successively pass by an index as the milled head is turned. A portion of the field of view on one side is cut off at right angles to the cobweb threads, by a scale formed of a thin plate of brass having notches at its edge, whose distance corresponds to that of the threads of the screw, every fifth notch being made deeper than the rest for the sake of ready enumeration. The object being brought into such a position that one of its edges seems to touch the stationary thread, the other thread is moved by the micrometer screw, until it appear to lie in contact with the other edge of the object; the number of entire divisions on the scale

The Stage-micrometer constructed by Fraunhofer is employed by many continental Microscopists; but it is subject to this disadvantage,—that any error in its performance is augmented by the whole magnifying power employed; whilst a like error in the Eyepiece-micrometer is increased by the magnifying power of the eye-piece alone.

shows how many complete turns of the screw must have been made in thus separating the threads; while the number to which the index points on the milled head, shows what fraction of a turn may have been made in addition. It is usual, by employing a screw of 100 threads to the inch, to give to each division of the scale the value of 1-100th of an inch, and to divide the milled head into 100 parts; but the absolute value of the divisions is of little consequence, since their micrometric value depends upon the objective with which the instrument may be employed. This must be determined by means of a ruled slip of glass laid upon the stage; and as the distance of the divisions even in the best ruled slip is by no means uniform,' it is advisable to take an average of several measurements, both upon different slips, and upon different parts of the same slip. Here the draw-tube will be of essential use, in enabling the microscopist to bring the value of the divisions of his Micrometer to even numbers. Thus, suppose that with a 1-4th-inch object-glass, the tube being pushed in, a separation of the lines by one entire turn and 37-100ths of another were needed to take in the space between two lines on the ruled slip, whose actual distance is 1-1000th of an inch; then it is obvious that 137 divisions on the milled head are equivalent with that power to a dimension of 1-1000th of an inch, or the value of each division is 1-137,000th of an inch. But as this is an awkward number for calculation, the magnifying power may be readily increased by means of the draw-tube, until the space of 1-1000th of an inch shall be represented by a separation of the cobweb threads to the extent of 150 divisions; thus giving to each division the much more convenient value of 1-150,000th of an inch. The Microscopist who applies himself to researches requiring micrometric measurement, should determine the value of his Micrometer with each of the objectives he is likely to use for the purpose; and should keep a table of these determinations, recording in each case the extent to which the tube has been drawn out, as marked by the graduated scale of inches which it should possess. The accuracy with which measurements may be made with this instrument, is not really quite so minute as it appears to be; for it is found practically that when the milled head is so graduated, that, by moving it through a single division, the cobweb threads are separated or approximated by no more than 1-10,000th of an inch, it needs to be moved through four divisions, for any change in the position of the threads to be made sensible to the eye. Consequently, if three entire turns, or 300 divisions, were found to separate the threads so far as to coincide with a distance of 1-1000th of an inch on the ruled glass under a 1-8th of an inch

Of the degree of this inequality, some idea may be formed from the statement of Hannover, that the value of the different divisions of a glass ruled by Chevalier to 1-100th of a millimetre, varied between the extreme ratios of 31-36, the mean of all being 34.