There are three of these glass tubes, smaller one than another, and the size of the object must direct which to use ; but. in general, the less room the creature has to move about in, the easier it may be magnified, and the quieter it will lie to be examined. Three loose glasses, viz. one plain, and two concave, belong also to the microscope, and are designed to confine objects, or place them upon occasionally.
The long steel wire, fig. 7, (with a pair of plyers at one end, and a nob of ivory at the other, to hold fast, or lay objects on), slips backwards or forwards in a short brass tube, whereto a button is fu tend, which fits into a little hole near the edge of the brass plate L, and then the object may be readily brought to a right position, and a light be cast upon it, either by the looking-glass underneath, or, if it be opaque, by the plano-convex lens, N: L, fig. 4, is a flat piece of ivory, called a slider, with four round holes through it, and objects placed in them, between Mus covy talcs, or isinglass, kept in by brass wires.
It is proper to have a number of these sliders, filled with curious objects, always ready, as well as some empty ones, for any new thing that offers. When made use of, thrust them between the brass rings of the contrivance on purpose for them, as 'hewn in fig. 4, which shoots into i the round hole M, in the centre of the brass plate, L; this keeps them steady, and, at the same time, permits them to be moved to and fro for • thorough exami nation.
The upper part of Plate Microscope, he. describes the construction of a micro scope, used for observing the divisions on mathematical instruments, and subdi viding them into smaller portions. The drawing was made from one of those used by Mr. Troughton in his instruments; and the position of four of them are shewn in the drawing of one of his astronomical circles, see Circular Instrument, fig. 1, an elevation sideways of the microscope ; fig. 2, a section, in the same direction ; fig. 3, a section endways, and four and five parts shewn separately.
The microscope is fixed to the instru ment it is applied to, by two nuts, a a, figs. 1 and 2, which work upon a male screw, cut on the external tube ; these nuts have a smaller part turned upon i them, which exactly fit into • circular hole in a piece of brass fixed to the in strument, and by screwing the nuts tight, the microscope is fixed, as shewn in the drawing of the astronomical circle before referred to. B, fig. 1 and 2, is the sliding tube containing two convex lenses, b d, fig. 2, whose combined foci fall upon the wires to be hereafter described; this tube slides in another, El, screwed into a thin, square box, E, which contains the wires and screw, and it is shewn opened in fir. 3 ; it has a square frame, fig. 5, sliding in it ; to this is affixed a very fine screw, which comes through the top of the box, and has a nut, F, screwed on It; with in the box, is a spring formed of steel wire, acting upon the frame, fig. 5, so as to draw it into the box; by this the shoul der of the nut, F, is forced down upon the top of the box, and all shake or looseness in the motion prevented.
The frame has two exceedingly fine wires stretched across it, as in the figure, and it is by these the divisions on the in. etrument are observed ; G is a conical tube, screwed into the principal tube of the instrument, with the object-glass at its end : the box, E, also contains a thin brass plate, fig. 4, which slides in it be neath the frame, fig. 5, and is moved when necessary by a small screw, a, go.
ing through the bottom of the box, whose action is counteracted by two thin slips of watch-spring, (seen in fig. 3;) this plate has an oblong hole through it to see through, and on one side it is out into fine notches, at such a distance apart that one turn of the nut F, when viewed through the eye-tube, moves the cross wires in the frame, 5, exactly one of these notch es, and by that means the notches regis ter the number of revolutions the'nut has made. In adjusting and adapting this Microscope to any instrument, the sliding eye-tube, B, is to be slid in or out of the tube D, till the cross wires in the frame, fig. 5, are seen perfectly distinct. The microscope is then to be placed in such a position on the instrument it is applied to, that the line of divisions on the arc shall be parallel to the motion of the cross wires and frame, fig. 5, and screwed into its holder by the nuts a a, (as shewn in the drawing of the circle) and the fo cus is adjusted so as to see the divisions on the arc distinctly, by turning these screws backwards or forwards, and mov ing the whole microscope nearer to or further from the arc, until it is adjusted to distinct vision. The operator then looks through the microscope, and observes whether one division on the di vided arc of the instrument answers to twenty of the notches of the scale, fig. 4, (which will each be then equal to one minute ;) if not, the conical tube, G, con taining the object-glassat its end, must be screwed in or out of the body of the microscope, until the image of the divi sion or degree is lengthened or shorten ed till it does, and a loose nut on the tube holds it any place required ; if this adjustment deranges the other before made for distinct vision, it must be recti fied by the nuts a a, and if this alters the space measured by the scale, fig. 4, the object-gbas must be altered, and then again the nuts, a a, first one and then the other, until both adjustments of measure and distinct vision are perfect. The small screw which gives motion to the scale, fig. 4, is used to adjust it to the point of commencement on the circle or divided arc ; or when two opposite microscopes are applied to the same circle, to adjust them so that they shall both read alike, that is, so that a line between them shall pass through the centre of the circle, and for the same purpose the small divided circle, r, of the not F, will slip round upon the nut when required, without turn ing the screw to bring the first division up on it to the index, 1, when the cross wires coincide with the point of commence ment of the circle or divided arc.
For reading the divisions by this mi croscope, the middle notch of the scale, fig. 4, is accounted the first, and every fifth is denoted by a longer notch, and eve. ry tenth by a still longer instead of figures. If now, when the circle is set to its re quired position, and observed through the microscope, any division or degree on the circle exactly coincides with the middle or first notch on the scale, the reading will be even degrees; if the di vision on the circle does not match with the first notch on the scale, the nut, F, of the screw must be turned, until the cross wires in the frame, fig. 5, exactly coincide with the division on the circle ; the number of notches on the scale de notes the minutes, and the number of the division on the small circle x, on the nut, F, which is opposite to the index, 4 denotes the number of seconds. See Orrice.