128 ACCESSORY APPARATUS. 



the object on the surface of the thin glass which covers it. The 

 readiest mode of doing this, when the object is large enough to 

 be distinguished by the naked eye, is to make a small ring round 

 it with a fine camel-hair pencil dipped in Indian ink ; but when 

 the object is not thus visible, the slide must be laid in position 

 on the stage, the object "found" in the microscope, the con- 

 denser adjusted to give a bright and defined circle of light, and 

 then, the microscope-body being withdrawn, the black ring is to 

 be marked around the illuminated spot. The same end, how- 

 ever, may be more precisely as well as more neatly accomplished, 

 by attaching an object-marker to the objective itself. That of 

 Mr. Tomes consists simply of an ivory cap, fitting over the l-4th- 

 inch objective, having its extremity narrowed down (like that of 

 the objective itself), but perforated in the centre, so as to form a 

 minute ring; the object having been "found" and brought into 

 the centre of the field, the cap is placed upon the objective, the 

 ring is blackened with Indian ink, and then, being carefully 

 brought by the focal adjustment into contact with the surface of 

 the glass, it stamps on this a minute circle enclosing the object. 

 A more elaborate contrivance of a similar kind, for marking a 

 circle round the object by a diamond point, attached to a cap 

 fitting on the objective, has been recently described by Mr. Bridg- 

 man (" Quarterly Microscopical Journal," vol. iii, p. 237) ; this 

 has the advantage of admitting a variation in the size of the 

 circle, and also of substituting a delicate line for the broad ring 

 which may partly obscure some neighboring object; but, on the 

 other hand, the very delicacy of the diamond marks prevents 

 them from being readily distinguished, and some kinds of glass 

 are so apt to "star" when marked with a diamond point, that 

 cracks or splinters may extend from the circle over the object it 

 is intended to indicate. The most unobjectionable and satisfac- 

 tory mode of "finding" an object, however, is, in the Author's 

 opinion, that which is aflbrded by a graduation of the movable 

 parts of the stage, in the manner to be presently described (§53). 

 52. Lever Stage. — Tlie general arrangement of the Traversing 

 Stage, now usually adapted to all high class Microscopes, has 

 been already explained (§§ 37-39) ; and though the details are 

 difi'erently constructed by the several makers, yet the general 

 principle is, that a lateral or horizontal movement is given to the 

 object-platform by one milled head, and a front to back or verti- 

 cal movement (the microscope being supposed to be placed in an 

 inclined position) by another. Thestage may be so constructed, 

 however, that motion shall be given to the object-platform by 

 means of a lever acting upon it in any required direetifcn ; this 

 being accomplished by making the object-platform slide laterally 

 on an intermediate plate, and by making the latter slide verti- 

 cally upon the fixed stage-plate which forms the basis of the 

 whole; each pair of plates being connected by dovetailed slides 

 and grooves. Thus the object-platform may be readily made to 



