MICRA [i MICRO-MILLIMETRES. 



231 



Royal Microscopical Society/ may be of use to the 

 reader : 



The stage micrometer cannot be used for direct measure- 

 ments with any save low powers, as it is necessary that the 

 lines be seen at the same time as the dimensions of the 

 object ; but indirectly very exact measurements may be 

 obtained. This may be accomplished by sketching the 

 outlines of the object lying upon the stage, using the 

 camera-lucida as shown in Fig. 189. The object is then to 

 be replaced by the stage micrometer, the lines of which are 

 drawn over the previous sketch, showing by simple inspec- 

 tion the magnitude of the object, or any part of it may 

 now be measured with a pair of compasses. 



It should be borne in mind that amplifications are gene- 

 rally expressed at a distance of 10 inches, and this is 

 accomplished by placing the drawing slab horizontally 

 10 inches beneath the acute angle of the camera-lucida 

 prism. If the object is required to be of the same size as 

 it appears upon looking into the microscope, the distance 

 of the drawing slab from the acute angle of the prism must 

 be equal to the distance of that point from the objective. In 

 estimating the magnifying power of an objective, the stage 



