70 THE MICROSCOPE. 



J-inch of Eoss, because we know its precise working and 

 defining power, and that most of the great achievements and 

 discoveries of the microscope have been made with powers 

 in no way superior to the last-mentioned J objective. The 

 many important contributions to microscopy by Owen, 

 Carpenter, Quekett, Ealfs, &c. have been made with 

 powers of limited capacity. 



MECHANICAL ARRANGEMENTS. 



Having explained the more important optical principles 

 of the achromatic compound microscope, it remains for 

 us to notice the mechanical and accessory arrangements 

 for giving these principles their full effect. In few philo- 

 sophical instruments is theoretical perfection so nearly 

 reached as in the microscope: in it the highest optical 

 skill is combined with the most consummate mechanical 

 contrivance, and the mechanism of the instrument is of 

 much more importance than might be imagined by those 

 who have not studied the subject. In the first place, steadi- 

 ness, or freedom from vibrations not equally communicated 

 to the object under examination, and to the object-glass or 

 lenses by which it is viewed, is a point of the utmost im- 

 portance. The various improvements which of late years 

 have combined to make the microscope the superior instru- 

 ment it is, may be classed under eight heads : 



1. The circular rack, which is placed immediately under 

 the stage, and which is capable of carrying it round, in 

 some instruments, the entire revolution of a circle, is a 

 very convenient movement for altering the angle at which 

 an object is viewed, without putting it out of field or focus; 

 it may also be used as a goniometer, for measuring crystals. 



2. The clamping arc, peculiar to Eoss's microscopes, fixes 

 the microscope at any inclination, even after the suspension 

 joint has become supple by long use. 



3. The advantage of the sub-stage for holding and ad- 

 justing, by universal motions, all the illuminating and 

 polarising apparatus placed beneath the object, can hardly 

 be overrated. 



4. The dark-ground illuminator is an excellent contri- 

 vance by which a bright light is apparently thrown on 

 semi-opaque objects, though really coming beneath them, 



