MECHANICAL ARRANGEMENTS. 61 



illuminated by an entire, or, as it may be termed, a solid 

 cone of light of much larger angle than that of the object- 

 glass. The extinction of an object by excess of illumina- 

 tion may be well illustrated by viewing with a one-inch 

 object-glass the Isthmia, illuminated by'Gillett's condenser. 

 When this is in focus, and its full aperture open, the 

 markings above described are wholly invisible ; but as the 

 aperture is successively diminished by the revolving dia- 

 phragm, the object becomes more and more distinct, and 

 is perfectly defined when the aperture of the illuminating 

 pencil is reduced to about 20. The same point may be 

 attained, although with much sacrifice of definition, by 

 gradually depressing the condenser, so that the rays may 

 diverge before they reach the object; and it may be 

 remarked, generally, that the definition of objects is 

 always most perfect when an illuminating pencil of suit- 

 able form is accurately adjusted to. focus, that is, so that 

 the source of light and the plane of vision may be conju- 

 gate foei of the illuminator. If a condenser of 120 

 aperture, or upwards, be used as an illuminator, the mark- 

 ings of Diatomacese will be scarcely distinguishable with 

 the best object-glass, the glare of the central rays over- 

 powering the structure of those that are more oblique. 1 



MECHANICAL ARRANGEMENTS. 



Having now explained the more important optical 

 principles of the achromatic compound microscope, it 

 remains for us to notice the mechanical and accessory 

 arrangements, for giving those principles their full effect. 

 The mechanism of a microscope is of much more import- 

 ance than might be imagined by those who have not 

 studied the subject. In the first place, steadiness, or 

 freedom from vibrations not equally communicated to the 

 object under examination and to the lenses by which it is 

 viewed, is a point of the utmost consequence. 



One of the best modes of mounting a compound micro- 

 scope is that shown, fig. 36, which, although it does not 



(1) Object-finder. It is a great saving of time to use an object-finder, when 

 very minute objects are not distinguishable by the naked eye. Many forms have 

 been suggested and described by Mr. Tyrrel. Mr. Bridgman, and Mr. T. E. 

 Amyot in the "Microscopical Journal" for 1855 and 1856. 



