CHAPTER III. 



ACCESSORY APPARATUS. 



In describing the various pieces of Accessory Apparatus with -which, 

 the Microscope may be furnished, it will be convenient 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 objects which are 

 under examination ; and secondly, to notice such as have for their 

 function to facilitate that examination, by enabling the Micro- 

 scopist to bring the Objects conveniently under his inspection. 



Section 1. Appendages to the Microscope. 



63. Draw-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 Objective 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 in- 

 creased with advantage to any considerable extent, yet, if the 

 corrections of the Object-glass have been perfectly adjusted, its 

 performance is not seriously impaired by a moderate lengthening 

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

 many occasions in which some amplification is desired, intermediate 

 between the powers furnished by any two Objectives. Thus if one 

 objective give a power of 80 diameters, and another a power 

 of 120, by using the first and drawing out 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 those details is very much inter- 

 fered with, if, through its not occupying 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 the Lieberkiihn 

 (§ 92), since, if any light be transmitted to the eye direct from the 



