APPARATUS FOR HOLDING SPECIMENS 



51 



FiQ. 44.— No. 3307, Sliding Ledge. 



amount equal to the field of view of the microscope, and the 

 specimens pushed all the way along. It can then be raised a 

 similar amount and 

 pushed back, and 

 80 on till the 

 whole area has 

 been searched. It 

 is not so conveni- 

 ent as a mechani- 

 cal stage, but 

 makes an inex- 

 pensive substitute. 



A mechanical 

 stage is an appa- 

 ratus which holds 

 a slide or object- 

 holder, and by 

 means of two 

 racks actuated by 

 milled heads moves 

 it in a delicate manner in either direction. One milled head 

 travels the object laterally, the other longitudinally. This 

 appliance is almost essential for the delicate movement of the 

 object when exacting work is being performed, and it has 

 other important uses. It enables the whole of a specimen to 

 be systematically examined over its entire surface step by 

 step, in a manner that is impossible by hand. It is provided 

 with scales and verniers, so that any position ia a specimen can 

 be recorded. The readings of the scale may be written upon a 

 label on the sKde, and the specimen found at any future time by 

 setting the stage to the same reading (see page 72). 



Fig. 94, page 99, shows a form of mechanical stage that is 

 very popular. It can be attached to a microscope and removed 

 at will, and it does not interfere with the adjustments of the 

 substage apparatus or alter the level of the stage. It consists 

 of a frame which holds the ends of a 3 x 1-inch shp, and moves 

 it on the flat stage of the microscope, with which the slip is always 

 in contact. A spring presses the slip down on to the stage and 

 may be turned aside when not required. It has a lateral travel 

 of 2^ inches (65 nam.) and a vertical travel of 1 inch (25 mm.) 



Fig. 45 shows a concentric rotating stage, with a mechanical Eotating 

 stage built into its surface. In this case the slide moves longitudin- ^S^*"'"*' 

 ally along the base-plate of the mechanical stage, but the whole 

 base-plate moves laterally to and fro. The mechanical portion of 

 the stage can be racked completely ofi the circular stage, leaving 

 a plain stage-plate for the examination of large objects ; but in 

 this case a small readjustment of substage apparatus is required 

 to compensate for the thickness of the travelling base-plate which 



