THE OBJECTIVE 123 



objective of 0.65 aperture is popular. The dry 40-times 

 fluorite or apochromatic objective of 0.85 or 0.95 aperture 

 is, it seems, to be avoided for ordinary purposes, unless 

 the user is ready to purchase only covers 0.17 milHmeter 

 thick, or to compensate for different cover-glass thicknesses 

 by adjusting the tube length, or by using a correction 

 collar. For the highest oil-immersion objective, the lower 

 aperture of 1.2 or 1.25 is to be favored for routine work, 

 instead of 1.3, because of the greater working distance, 

 and other advantages. An increase of more than 0.1 of 

 working aperture can readily be obtained for the objective 

 of 1.2 aperture by the use of a corrected, instead of an 

 uncorrected, condenser, and by employing screens to 

 moderate too intense hght, instead of having to lower the 

 working aperture by closing up the condenser iris. 



Of achromatic and fluorite objectives used on a microscope 

 having a double nosepiece, instead of objectives 10 and 

 40 (the usual outfit), the objectives 10 and 20, or 10 and 

 50 oil, seem useful combinations. With a triple nosepiece 

 on the microscope, instead of the usual objectives, 10, 

 40, and 90 or 100 oil; the objectives 10, 20, and 50 oil; or 10, 

 50 oil and 90 oil, appear to the writer to be good combina- 

 tions. Lastly, with a microscope provided with a quad- 

 ruple nosepiece, the objectives 10, 20, 50 oil, and 90 oil; 

 or 10, 40 dry with collar, 90 water immersion and 100 

 fluorite; are apparently suitable for biological work with 

 high powers. Some may prefer to use the objective 40 

 of 0.65 aperture, having No. 1 cover-glasses and adjusting 

 the objective when necessary by altering the tube length; 

 and some may prefer objective 40 of 0.85 or 0.95 aperture 

 with a correction collar; though the writer has tried these, 

 and does not hke them for continuous work, where every 

 item of labor saving counts, for the collar of a dry objective 

 on a balsam preparation requires constant changing, 

 varying with the depth of the part of the object viewed. 

 It is advisable, however, that the nosepiece should take 

 in all the objectives likely to be frequently used; for, with 

 much screwing and unscrewing, time is wasted, and objec- 



