THE OBJECTIVE 117 



But a change of cover thickness may be partly balanced by 

 a change of tube length, or a change of lens distance. Not- 

 withstanding this, slightly better images are given (in the 

 writer's experience) by standard cover thickness, standard 

 tube length, and standard lens distance, than result from 

 any corrections. 



A high-power microscope objective is one of the finest 

 optical products, ranking near the astronomical telescope 

 objective in accuracy of manufacture, and the microsco- 

 pist's endeavor should be to get out of it all the possibilities 

 the optician put in. 



Focal Lengths. — The lowest ordinary single objectives 

 of 50 to 25 millimeters focal length are probably best 

 replaced by the twin-objective binocular, which gives 

 marked stereoscopic images; for stereoscopic vision is 

 at its best at magnifications below 50 or 100. Where the 

 twin-objective binocular usually leaves off, namely at useful 

 magnifications of 50 to 100, there the single-objective 

 binocular may well begin. Lower magnifications (from 30 

 upwards) can be had, however, with the 10-times objective 

 on the monocular or on the standard monobjective bin- 

 ocular, by using low eyepieces. Hence there would 

 seem to be little use, in advanced work, for single objectives 

 lower than the 16-millimeter (or of 8 to 10 initial magnifica- 

 tion), and such objectives are not usually made in the 

 apochromatic series. Their employment would require 

 the removal of the condenser; whereas the 16-millimeter 

 objective can be used with the same condenser as the high- 

 est oil-immersion objective, with a sufficiently large source 

 of light, if care is taken to reduce the aperture of the con- 

 denser enough every time. Objectives may ordinarily 

 be classed, in high-power work, as searching or viewing, and 

 observing or working, objectives. 



The 2^_inch or 16-millimeter objective, of about 10 

 initial magnification (new style of reckoning) with an 

 aperture of 0.2 to 0.3, will give useful magnifications from 

 50 to 200 times, and is usually the finder or searcher objec- 

 tive. The 3^^-inch or 8-millimeter objective of 20 initial 



