348 



MICROSCOPE. 



should be paid to the action of this rack ; it 

 should work through a triangular socket at the 

 top of the pillar of at least an inch long, closely 

 embracing the bar, so that it may be moved up 

 and down without the least shake, or variation 

 from its axis, or tendency to slip, when loaded 

 with the full weight it has to carry; at the 

 same time, it should be sufficiently sensitive to 

 be moved by the slightest rotation of the pinion, 

 which may be effected, not only by the milled 

 head, but by a lever, as in Mr. Holland's mi- 

 croscope. Such a rack will afford the means 

 of focal adjustment for doublets and triplets of 

 high power, as well as for ordinary magnifiers. 

 If it be considered desirable to vary the length 

 of the pillar, and consequently the height of 

 the stage, this may be effected by attaching the 

 rack, stage, Sec., to a piece of tube which shall 

 slide within the one that is attached to the 

 base ; if the top of the lower one is sp?'uns, and 

 surrounded by a clamping screw, the upper one 

 may be firmly fixed at any elevation. This 

 plan has been proposed by Dr. Goring for the 

 pillar of his engiscope; but we consider it per- 

 fectly inapplicable to a large instrument, of 

 \vhkh the pillar should be solid, in order to 

 avoid oscillation. The triangular bar carries at 

 its top a double arm, attached to it with a 

 screw and a small milled head, by which its 

 movements may be rendered more or less free. 

 This arm should be thick and well hammered, 

 in order that, when it carries the compound 

 body, it may not be subject to vibration. The 

 object of the double bend in one side of it is to 

 allow the nose of the body, which is screwed 

 into the hole on that side, to project below it, so 

 that the magnifiers may be attached on its un- 

 der side ; by this means, the necessity for un- 

 screwing the body every time that the magni- 

 fier is to be changed (which is requisite where 

 the magnifier is screwed into the arm and the 

 body into it) is avoided, without any increase 

 in the length of the rack, which would other- 

 wise be necessary for low powers. The other 

 side of the arm carries the magnifiers, when 

 employed as single lenses ; the same set may 

 be used as are fitted to the compound body, 

 their interior being screwed for attachment to 

 it, and their exterior being adapted to drop 

 loosely into the hole of the second arm. For 

 such a microscope, we should recommend a 

 series of ordinary lenses from 2^ or 2 inches 

 focus to ith of an inch, and two or three dou- 

 blets from T ' 5 th to ^th, with a triplet of 3 Lth. 

 These may all be used as objectives with a 

 meniscus eye-piece ; though, for the latter, Mr. 

 Holland's will, in most cases, be preferable. 

 The stage of this microscope may be a simple 

 plate, attached to the top of the pillar, as in 

 Jig. 166, with a spring stage and other appurte- 

 nances adapted to it. We are inclined to re- 

 commend for such a microscope, however, for 

 the sake of simplicity and facility of use, a 

 stage in which the ordinary principle of fixity 

 is in some degree departed from. The upper 

 part of it consists of a thick and strong fork, 

 projecting about 2 inches and about 1 \ inch 

 broad, of well-hammered brass, firmly attached 



to the top of the pillar; at about Jths of an inch 

 below this is a plate of similar dimensions, 

 with an aperture of about an inch. Bearing 

 upon this lower plate by a spiral spring, and 

 guided by vertical pins working through aper- 

 tures in it, is a thin moveable plate, which is 

 constantly being pressed upwards by the spring 

 against the fork. This stage combines, in a 

 degree which renders it an extremely conve- 

 nient one, the advantages of the ordinary spring- 

 stage, and the fork. The objects, whether 

 placed in sliders, on slips of glass, or in aquatic 

 boxes, are readily slid under the prongs of the 

 fork (which should be bevelled off on the under 

 side so as to allow them to enter), and are held 

 by the spring with sufficient firmness in any 

 position, whilst ready movement is also per- 

 mitted them. The Ibrk may have a vertical 

 hole drilled in it on each side, for holding the 

 stage forceps or other such instrument; and it 

 may be made thick enough to allow of a la- 

 teral hole for fixing the condenser, without 

 being too much weakened. The thickness 

 of the fork will be of no kind of inconve- 

 nience, as the nose of the microscope will have 

 free play from side to side between its prongs. 

 To the lower plate may be attached ground- 

 glass, stops, condensers, polarising apparatus, 

 or any other required fittings. The disadvan- 

 tage of this stage is that it affords no firm sup- 

 port to the object, which is in some instances 

 of importance. This may be obtained by sim- 

 ply adapting a plate to the upper side of the 

 fork, which may be received into grooves on its 

 lower surface ; and this plate may then be con- 

 sidered as the stage. Whenever such an in- 

 strument is being adapted to the purpose of 

 dissection, all that is necessary is to raise up a 

 support for the hands on each side of the stage, 

 which may consist of books, or still better of 

 blocks of wood cut to the form of the outside 

 of Mr. Holland's case; and the height of the 

 stage being then adjustible by the sliding tube 

 of the pillar, all the advantage of Mr. Holland's 

 microscope, exceplilsself-containedness, may be 

 secured. We have only further to mention 

 the mirror, which should be at least two inches 

 in diameter, and have one of its sides plane, 

 and the other concave. Its stem should be 

 attached to a short piece of sprung tube sliding 

 over the pillar, and capable of being secured 

 in any situation by a clamping screw. 



If a still more portable microscope be re- 

 quired, combining considerable range of power 

 with great exactness of adjustment, we can 

 strongly recommend an instrument constructed 

 upon the plan of the large one subsequently to 

 be described, (Jig. 167,) but upon about two- 

 fifths of the scale. Its pillar may either be 

 screwed into the lid of the box containing the 

 apparatus, or mounted on a tripod similar to 

 that which is used for portable telescopes, the 

 tripod being reversed, and the legs being folded 

 round the stem, when it is packed in its box. 

 The delicate focal adjustment of which this 

 microscope is susceptible renders it more ad- 

 vantageous than the one last described, when 

 deep magnifying powers are being employed; 



