94 



A sub-stage may, in a general way, be said to be 

 preferable when it is adjustable, particularly in the 

 use of a condenser. It is absolutely necessary to 

 adjust the condenser for different objectives and this 

 must be done so nicely that a sliding movement is 

 hardly sufficient. In biological work the condenser 

 is at the present day a necessity, and this is now con- 

 structed as a separate attachment with all the neces- 

 sary adjustments, in which the rack and pinion is 

 deemed of first importance. 



Objectives and Eye-Pieces. It is hoped 

 that the information given of the various qualities in 

 an objective will aid to make a suitable selection of 

 the optical parts. As the stands have been classified 

 in long and short standard tubes, the first quality to 

 look for is, after the stand has been selected, their 

 suitability to it. 



As will have been seen under the proper head, a 

 variety of powers is obtained by a suitable combina- 

 tion of eye-pieces and objectives, and while power 

 alone can be obtained by increasing the power of the 

 eye-piece, it is not advantageous to do so. For ordi- 

 nary work no higher eye-piece than a | should be 

 used. In catalogues many outfits are made up of one 

 eye-piece and two objectives, but this is only for the 

 purpose of reducing price to a minimum. It is always 

 advisable, when means will permit, to select two eye- 

 pieces, preferably the 2 inch and 1 inch, and insist 

 they be par-focal, as this will be found extremely 

 convenient and will not disturb the optical standard 



