ACCESSORY APPARATUS. 



87 



eye-piece; counteracting its aberrations by substituting a convexo-concave 

 lens for the plano-convex which forms the field-glass of the ordinary 

 Huyghenian eye-piece. 1 



83. 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 ( 25). For although, as a general rule, the magni- 

 fying power cannot be thus increased with advantage to any considerable 

 extent, yet, if the corrections of low objectives have been well adjusted, 

 their performance is not seriously impaired by a moderate lengthening of 

 the body; and recourse may be conveniently had to this on many occa- 

 sions 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; so as to prevent the vividness and distinct- 

 ness of its image from being interfered with by extraneous light. In the 

 use of the Micrometric eye-pieces to be presently described ( 90, 91), 

 very great advantage is to be derived from the assistance of the Draw- 

 tube; as enabling us to make a precise adjustment between the divisions 

 of the Stage-micrometer and those of the Eye-piece micrometer; and as 

 admitting the establishment of a more convenient numerical relation be- 

 tween the two, than could be otherwise secured without far more elabo- 

 rate contrivances. Moreover, if, for the sake of saving room in packing, 

 it be desired to reduce the length of the body, the draw-tube (in a Mono- 

 cular Microscope) affords a ready means of doing so. Objectives of high 

 power, however, require special adjustment when any considerable length 

 of Draw- tube is used. FIG. 59. 



84. Lister's ^rector. This instrument, first applied to 

 the Compound Microscope by Mr. Lister, consists of a tube 

 about three inches long, having a meniscus at one end and a 

 plano-convex lens at the other (the convex sides being upwards 

 in each case), with a diaphragm nearly half way between them; 

 %nd this is screwed into the lower end of the draw-tube, as 

 shown in Fig. 59. Its effect is (like the corresponding erector 

 of the Telescope), to antagonize the inversion of the image 

 formed by the object-glass by producing a second inversion, 

 so as to make the Image presented to the eye correspond in 

 position with the Object an arrangement of great service in 

 cases in which the object has to be subjected to any kind of 

 manipulation. The passage of the rays through two addi- 

 tional lenses of course occasions a certain loss of light and 

 impairment of the distinctness of the image; but this need 

 not be an obstacle to its use for the class of purposes for 

 which it is especially adapted in other respects, since these 

 seldom require a very high degree of defining power. By the 

 position given to the Erector, it is made subservient to an- 

 other purpose of great utility; namely, the procuring a very 

 extensive range of Magnifying power, without any change in 

 the Objective. For when the draw-tube, with the erector fit- 



Erector. 



Op. cit., Vol. v. (1880), p. 81. 



