64 Modern Microscopy. 



The Measurement of Objects. 



There are several ways in which this may be effected : 



1. By having the stage divided applicable to mechani- 



cal stages only. 



2. By means of a camera lucida and a stage micro- 



meter. 



3. By means of eyepiece and stage micrometers. 



1. If the movements of a mechanical stage are divided 

 and read by verniers to very small parts of an inch or 

 millimetre, the measurement of an object can be effected by 

 having in the eyepiece a disc of glass with a diamond-cut 

 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. Then again read the stage divisions, and by 

 subtracting one from the other the measurement will be 

 ascertained. For quick work, and without extraneous 

 appliances, this is accurate, and largely used. 



2. Camera Lucida and Stage Micrometer. A stage 

 micrometer usually consists of a number of lines equi- 

 distant at the J^-Q- r roW f an inch, or the T V 

 and r ^-Q of a millimetre. This is put on the stage and 

 focussed like an ordinary object. The camera lucida is then 

 fixed on to the eyepiece, and the lines are projected on to a 

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

 explained on page 62. The lines so projected are then 

 measured, and supposing the lines of the micrometer, which 

 are T -o 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, and 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 



