62 THE MICROSCOPE 



Fo^ each of these methods a stage micrometer is 

 necessary. This is a 3 by i inch glass slip having en- 

 graved on it a scale divided to hundredths of a milli- 

 metre (o.oi mm.), every tenth line being made longer 

 than the intervening ones, to facilitate counting; and 

 from these engraved lines the measurement in every 

 case is evaluated. A cover-glass is cemented over 

 the scale to protect it from injury. 



FIG. 55. Camera lucida, Abbe pattern. 



(a) By means of the Camera Lucida. 



1. Attach a camera lucida (of the Wollaston, Beale, 

 or Abbe pattern) (Fig. 55) to the eyepiece of the 

 microscope. 



2. Adjust the micrometer on the stage of the micro- 

 scope and accurately focus the divisions. 



3. Project the scale of the stage micrometer on to a 

 piece of paper and with pen or pencil sketch in the 

 magnified image, each division of which corresponds 

 to 10 fi. Mark on the paper the optical combination 

 (ocular objective and tube length) employed to pro- 

 duce this particular magnification. 



