THE MICROSCOPE AND ITS ACCESSORIES. 9 



Abbe's condenser. The object is then seen brilliantly illuminated 

 on a black background. 



8. Objectives. For ordinary work every microscope requires to 

 have two objectives of different magnifying powers ; one of these, 

 when used with an ordinary ocular, should magnify about 60-75 



FIG. 12. Cylindrical Diaphragms 

 (i, 2, 3), a central stop-dia- 

 phragm (5). 



FIG. ii. Iris Diaphragm, showing a small 

 aperture. 



diameters linear. This is spoken of as the low power, and in these 

 pages is indicated by the letter (L). The other should magnify 

 from 350-400 diameters linear, and is called the high power, 

 indicated by the letter (H). 



If an English make of lens is preferred, let them be an " inch " 

 and a " quarter inch." The term " one-inch objective " has no direct 

 relation to the distance between the object and the lens, but indi- 

 cates that such a lens possesses the same magnifying power as a single 

 lens of one-inch focus. Swift's new high-angled i-inch lens is a 

 very good low-power lens. If Continental lenses be preferred, and 

 if Zeiss's be selected, let them be A and D, equal to -inch and 

 J-inch respectively. Zeiss makes lenses AA andDD of slightly better 

 quality, which cost a few shillings more. If Leitz's lenses be pre- 

 ferred, use Nos. 3 and 7 ; and if Hartnack's, Nos. 3 and 7 3 being 

 the weaker lens. Crouch's lenses are excellent. 



Lcitz numbers his objectives 1-9, and Zeiss A-F. i and A are the 

 weakest objectives, and the magnifying power increases up to 9 and F. 



The microscopes of Reichert of Vienna are also excellent. The 

 stands arc all provided with a universal thread or screw, so that the 

 lenses of Zeiss, of English makers, or of others may be adapted to 

 them. 



For certain special purposes much higher powers are required, 

 but for ordinary work these lenses are sufficient. 

 3 



