130 THE USE OF THE MICROSCOPE 



8. The oil-immersion 60 (3 millimeters) is sometimes to be 

 preferred to the oil-immersion objective 90 (2 millimeters) of the 

 same aperture, having a longer working distance, and other 

 desirable qualities. It is however more sensitive to tube-length 

 differences. 



9. The high dry objective, achromatic or apochromatic, and 

 the high water-immersion, should have correction collars, 

 especially if used on a binocular. 



10. For routine work, use, preferably, oil-immersion objectives 

 of apertures 0.85 to 1.2, rather than those of 1.3 aperture. The 

 former are often useful in investigation also, when it is not a 

 question of markings or bodies at the limit of vision. 



11. Do not dismount the lenses of the objectives by removing 

 the lens rings from the objective tube in the new form of mount, 

 or the centering may be injured. 



12. Keep the front lens of the objective optically clean. 



13. In summer, or in damp climates, it may help to keep the 

 objectives in a dessicator. 



14. One who has acquired some skill in microscopical matters 

 can correct (for covers under 0.17 millimeter thick) a high dry 

 objective, or a high water-immersion objective, without correc- 

 tion collars, by unscrewing slightly the front lens or the two 

 combined front lenses. This is especially important with the 

 binocular microscope. 



15. One should get the most out of every objective, by using a 

 large enough condenser cone. 



16. All objectives should stand at least a % cone (that is a 

 cone of J^8 the diameter of their back lens) of light from the con- 

 denser, on a well-stained object, without glare. This cannot be 

 done if there is noticeable spherical aberration due to maladjust- 

 ment, or if they are out of center on the nosepiece, or if the con- 

 denser is not concentric with them. 



17. The condenser light circle on the back of the focused 

 objective should be under frequent observation. 



18. The microscope objectives made by the leading makers, 

 when correctly used, usually show differences less than identical 

 objectives might show in the hands of skilled and unskilled users. 



