NOSE-PIECE. — OBJECT-MAKKEB. 108 



this Nose-piece the arm is straight ; and its use is attended with 

 the inconvenience of often bringing down upon the Stage the Objec- 

 tive not in use, unless the relative lengths of the two objectives are 

 specially adjusted to prevent 

 this. This inconvenience is 

 still more felt in triple and 

 quadruple nose-pieces. It is 

 avoided, however, in the con- 

 struction adopted by Messrs. 

 Powell and Lealand (Fig. 56), 

 and by MM. Nachet ; the 

 bend given to the arm having 

 the effect of carrying the 

 Objective not in use com- 

 pletely off the Stage. — The 

 working Microaoopist will *<""* **£££•***••** "* 

 scarcely find any Accessory 



more practically useful to him than this simple piece of apparatus. 

 74. Object- Marker. — All Microscopists occasionally, and some 

 continually, feel the need of a ready means of finding, upon a glass 

 slide, the particular object, or portion of an object, which they 

 desire to bring into view ; and various contrivances have been 

 suggested for the purpose. Where different magnifying powers 

 can be readily substituted one for another, as by the use of the 

 Erector (§ 64) or of the Nose-piece (§ 73), no special means are 

 required ; since when the object has been found by a low power, 

 and brought into the centre of the field, it is rightly placed for 

 examination by any other Objective. Even this slight trouble, 

 however, may be saved by the adoption of more special methods ; 

 among the simplest of which is marking the position of 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 or under the Simple Microscope, 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 Micro- 

 scope, the Condenser 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 light, and then, the Microscope-body being withdrawn, the black 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, 

 however, may be more precisely as well as more neatly accom- 

 plished, 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 

 it blackened with Indian -ink, and then, being carefully brought 



