96 HOW TO SEE WITH THE MICROSCOPE. 



keep the base always within the circle drawn on the 

 paper, and continue the movement (either to the right 

 or left, as the case may be), until one-half of the field 

 shall be bisected i. e., one-half bright and one-half in 

 the shade. Now select some straight portion of the 

 base, marking a straight line coincident therewith with 

 a pencil, or, placing a rule in contact with any two 

 salient angles of the base, draw a line. 



Next, revolve the entire stand in the contrary direc- 

 tion, passing the candle, and until you again get a field 

 half bisected similarly, as before mentioned. Now 

 selecting the same side, or the same two salient corners 

 of the stand, mark another line. 



These two lines will be found divergent with each 

 other, and with a parallel rule it will be found easy to 

 produce parallel lines which can be continued until 

 they meet ; this done, it only remains to measure the 

 angle obtained with a common protractor, or other in- 

 strument designed for the measurement of angles. 



Where tolerable accuracy is important it will be well 

 to cross-question the result by repeating the operation. 

 There ought not to be a discrepancy of more than one- 

 half of one degree. 



Those who possess the large stands fitted with gradu- 

 ated and revolving platform will of course avail them- 

 selves of the convenience ; the operation is practically 

 the same; thus, having bisected the field (say to the 

 right), read the angle, which will be indicated by a 

 mark on the platform adjacent to the graduations ; then 

 bisect the field to the left, read the angle again, sub- 



