select two eyepieces, preferably the 2 inch and i inch 

 and insist that they be parfocal, as this will be found 

 extremely convenient and will not disturb the optical 

 standard length. If for any work 1-2 inch or higher ' 

 powers are desired, the solid eyepiece may be recom- 

 mended. With the apochromatic objectives use the 

 compensating eyepieces only. Every eyepiece should 

 be marked with its equivalent focus. 



Objectives. For all ordinary student and pro- 

 fessional work, not involving bacteriological investiga- 

 tions, the 3-4 inch 0.22 N. A., and 1-5 inch 0.62 N. A. 

 for long tube ; and 2-3 inch 0.24 N. A., and t-6 inch 

 0.85 N. A. or 1-8 inch 0.93 N. A., for short tube have 

 generally been accepted as best suited. 



Bacteriological investigations absolutely require an 

 oil immersion objective in which the 1-12 inch of 1.32 

 N. A. is generally employed. 



Botanical work necessitates a 2 inch or 3 inch in 

 addition to the regular outfit. 



For urinary work and blood counting where a 

 considerable working distance is an absolute necessity 

 the 1-5 or 1-6 inch of reduced aperture and long working 

 distance are recommended. 



Objectives of Wide Aperture. It will be 

 noted that objectives of the lowest price and lowest in 

 the scale of efficiency have been recommended as 

 ample for ordinary use, but it is w r ell to bear in mind 

 or study the advantage which is obtained by objectives 



190 



