the optical parts. Since the stands have been 

 classified as of long and short standard tube 

 lengths, the first quality to look for is, after the 

 stand has been selected, the suitability of objec- 

 tive and eyepiece to it and to the work. As a 

 variety of powers is obtained by a suitable combi- 

 nation of eyepieces and objectives and while power 

 in itself can be obtained by increasing the power 

 of eyepiece, this is not advantageous as we have 

 shown. 



Eyepiece. Select the Huyghenian eyepiece 

 and of no higher power than f inch. In cata- 

 logues many outfits are made up of one eyepiece 

 and two objectives, but this is only for economy ; 

 it is always advisable to select two eyepieces, per- 

 ferably the -2 inch and 1 inch and insist that they 

 be parfocal, as this will be found extremely con- 

 venient and will not disturb the optical standard 

 length. If for any work | inch or higher powers 

 are desired, the solid eyepiece may be recom- 

 mended. With the apochromatic objectives use 

 the compensating eyepiece only. Every eyepiece 

 should be marked with its equivalent focus. 



Objectives. For all ordinary student and pro- 

 fessional work, not involving bacteriological inves- 

 tigations, the |- inch 0.24 N. A., and inch ().<>2 

 N. A. for long tube ; and f inch 0.25 N. A., and 

 inch 0.82 N. A. or | inch 0.85 N. A., for short tube 

 have generally been accepted as best suited. 



