Accessory Apparahis 115 



line across the centre. The object that it is desired to 

 measure is set with one point exactly against the diamond- 

 cut line, which, of course, will appear in the field, and the 

 reading of the stage divisions taken. The stage is then 

 slowly moved along by means of the milled head until the 

 other edge of the specimen to be measured is exactly touch- 

 ing the line. The readings of the stage divisions are again 

 taken, and by subtracting one from the other the measure- 

 ment will be ascertained. For quick work, and without 

 extraneous appliances, this is fairly accurate, and largely 

 used. 



2. Camera Lucida and Stage Micrometer. — A stage micro- 

 meter usually consists of a number of lines photographed, 

 or ruled with a diamond on a slip of glass to the scale of 

 xBTj-th or iTnr<r^'h part of an inch, or the xVth and rrirth of a 

 millimetre. This is put on the stage and focused like an 

 ordinary object. The camera lucida is then fixed to the 

 eyepiece, and the micrometer lines are projected on to a 

 piece of paper in the same way as when drawing an object 

 explained on page 112. The lines so projected are then 

 measured, and supposing the lines of the micrometer, which 

 are x^Tj-th of an inch apart, appear when drawn on the paper 

 1 inch apart, it is at once known that the magnifying power 

 in use is 100 diameters. The object may be measured in 

 the same manner. Measurements should be taken about 

 the centre of the field, and not towards the edge, especially 

 with high powers, as, owing to curvature of the field, the 

 outer edges appear more highly magnified than the centre. 



3. The Eyepiece Micrometer and Stage Micrometer. — The 

 stage micrometer, as previously described, is placed on the 

 stage, and a somewhat similar micrometer is put into the 

 eyepiece. This latter is generally divided into hundredths 

 of an inch, but no exact value is needful so long as the lines 

 are equidistant. On focusing the stage micrometer the 

 two sets of lines will appear in the field at once. It is now 

 desirable to ascertain how many divisions of the eyepiece 

 micrometer are included between one of the spaces-r-that 



8—2 



