282 USE OF THE MICROSCOPE 



of the drawing pencil can be seen with equal clearness. It is 

 best to begin the adjustment with the free opening in both dia- 

 phragms in position, that is, with no smoked glass intervening 

 between drawing paper or object, and then if the object, for 

 instance, should be too bright the diaphragm relating to it can 

 be revolved until this is corrected, or if the drawing paper is too 

 bright its diaphragm is to be revolved until the right degree of 

 smoked glass is in position. 



In drawing from the camera lucida it is a convenience to have 

 a drawing board (Fig. 152) adjusted to the same height as the 

 microscope stage, and if the microscope is used tilted the drawing 

 board should be set to the same angle as the stage. The mirror 

 should be adjusted to bring the center of the drawing vertically 

 below the middle part of the mirror, for the projection of the 

 image off to one side of the mirror causes its distortion. 



The determination of the magnification of a drawing done 

 with a camera lucida is made by projecting the scale of a stage 

 micrometer upon the drawing paper by means of the camera 

 lucida and drawing it there and measuring the drawing with 

 a millimeter scale. Then the magnification is obtained by 

 dividing the value of the magnified drawing by the actual value 

 of the scale. For example, if one of the finest divisions of the 

 micrometer scale (.01 mm.) measures 5 mm. in the drawing the 

 magnification would be .ot=5oo- Of course the magnification 

 would have to be determined for each objective used and the 

 tube length must be kept the same for the micrometer scale as 

 for the object, and the distance of the drawing paper below the 

 mirror must be kept the same. 



Use of the Polariscope. — The polariscope is very useful in 

 detecting the presence of minute starch grains and crystals and 

 in bringing out sharply fragments of sclerenchyma tissues in 

 powdered drugs, etc. The polariscope consists of the polarizer 

 which is placed beneath the stage, and the analyzer surmounting 

 the eyepiece. Proceed with the polarizer as follows: Adjust the 

 mirror so as to reflect light into the microscope, and looking into 

 the eyepiece rotate the analyzer. It will be seen that during this 



