90 



APPENDAGES TO THE MICROSCOPE. 



Fig. 46. 



mirror, in consequence of the disk failing to occupy the entire 

 field, it greatly interferes with the vividness and distinctness of 

 the image of the object. In the use of the Micrometric eye- 

 jneces to be presently described (§§ 67, 68), 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 between the two, than could be otherwise secured without 

 far more elaborate contrivances. Moreover, if, for the sake of 

 saving room in packing, it be desired to reduce the length of the 

 body, the draw-tube affords a ready means of doing so ; since the 

 body may be made to 'shut up,' like a Telescope, to little more 

 than half its length, without any impairment of the optical per- 

 formance of the instrument when mounted for use (§ 43). 



64. Listers Erector. — It is only, however, in the use of the 

 Erector, that the value of the Draw-tube 

 comes to be fully appreciated. 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 ; and this is screwed 

 into the lower end of the draw-tube, as shown 

 in Fig. 46. Its effect is (like the corresponding 

 erector of the Telescope), to antagonize the inver- 

 sion 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 additional lenses of course 

 occasions a certain loss of light by reflexion 

 from their surfaces, besides subjecting them to 

 aberrations whereby the distinctness of the image 

 is somewhat impaired ; 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 another pur- 

 pose of great utility ; namely, the procuring a 

 very extensive range of Magnifying power, with- 

 out any change in the Objective. For when the draw-tube, 

 with the erector fitted to it, is completely pushed-in, the 

 acting length of the body (so to speak) is so greatly reduced 



Draw-tube fitted 

 with Erector. 



