CHAPTER III. 



ACCESSORY APPARATUS. 



42. In describing the various pieces of accessory apparatus 

 with which the Microscope may be furnished, it will be con- 

 venient in the first place to treat of those which form (when in 

 use) part of the instrument itself, being Appendages either to its 

 Body or to its Stage, or serving for the Illumination of the ob- 

 jects Avhich are under examination ; and secondly to notice such 

 as have for their function to facilitate that examination, by en- 

 abling the microscopist to bring the Objects conveniently under 

 his inspection. 



Section 1. Appendages to the Microscope. 



43. Draiv- Tube. — It is advantageous for many purposes, that 

 the Eye-piece should be fitted, not at once into the " body" of 

 the Microscope, but into an intermediate tube ; the drawing out 

 of which, by augmenting the distance between the object-glass 

 and the image which it forms in the focus of the eye-glass, still 

 further augments the size of the image in relation to that of the 

 object (§ 20). For although the magnifying power cannot be 

 thus increased with advantage to any considerable extent, yet, if 

 the corrections of the object-glass have been perfectly adjusted, 

 its performance is not seriously impaired bj' a moderate lengthen- 

 ing of the body ; and this may be conveniently had recourse to on 

 many occasions, in which some amplification is desired, inter- 

 mediate between the powers furnished by an}' two objectives. 

 Thus if one object-glass give a power of 80 diameters, and 

 another a power of 120, by using the first and drawing the eye- 

 piece, its power may be increased to 100. Again, it is often very 

 useful to make the object fill up the whole, or nearly the whole, 

 of the field of view : thus if an object that is being viewed by 

 transmitted rays, is so far from transparent as to require a strong 

 light to render its details visible, the distinctness of its details is 

 very much interfered with, if, through its not occujiying the 

 peripheral part of the field, a glare of light enter the eye around 

 its margin ; and the importance of this adjustment is even greater, 

 if opaque objects mounted on black disks are being viewed by 



