CH. X] DRAWING WITH HIGH POWERS 337 



a plano-concave or double-concave lens. This is mounted in a 

 fork-like holder and is set in the socket for the mirror stem of the 

 microscope (fig. 152, 187). Then the microscope is pushed up 

 toward the condenser until the parallel beam is of sufficient diam- 

 eter to fill the substage condenser. The substage condenser 

 diaphragm is opened to its full extent. 



479. Concave lens to be employed. This depends upon the 

 focus of the main condenser. If the focus is about 15 cm. (6 in.), use 

 a concave lens of -16 to -20 diopters ( 356). If the main condenser 

 has a focus of 20 to 40 cm. (8 to 16 in.), use a concave lens of -8 to 

 -12 diopters. The longer the focus of the main condenser the. 

 shallower can be the concavity of the parallelizing lens. Indeed, 

 for objectives of 16, 12, 10, and 8 mm. focus a condenser lens of 25 

 to 38 cm. (10 to 15 inches) focus gives very good results, when 

 the substage condenser is used without any parallelizing lens 

 (fig. 154). 



480. Position of the substage condenser; opening of the 

 condenser diaphragm. As pointed out in Ch. IX ( 407), the posi- 

 tion of the substage condenser must be very precisely determined 

 for different objectives and for different thickness of slides. 



To begin with, the substage condenser diaphragm is opened to 

 its full extent. Then in each case one must get the sharpest possi- 

 ble image by getting the best position of the substage condenser, 

 and closing the diaphragm more or less. As a general statement, 

 the diaphragm should be considerably wider open for drawing 

 than for ordinary observation. 



481. Oculars to employ for drawing. Those of X2, X3, X4, x6, 



x8, and xi2 may be used. Naturally, the lower and medium 

 powers give the more brilliant images as for direct observation, 

 One will rarely need to use an ocular higher than x8. 



482. Mirror or prism for reflecting the image-forming rays 

 down upon the drawing surface. For high power drawing it is 

 better to have the reflecting mirror or prism close to the ocular 

 (fig. 192) rather than to have it distant, as with the drawing table 

 in figure 182. 



