DRAW-TUBE — EEECTOE. 



119 



Fir. 32, 



i 



i 



the Lieberkiilin (§ 65), since if any light be transmitted to the 

 eye direct from the mirror, in consequence of the disk failing to 

 occupy the centre field, it greatly interferes with the vividness 

 and distinctness of the image of the object. In the use of the 

 Micrometric eye-pieces to be presently described (§§ 45, 46), 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 mi- 

 crometer ; and as . admitting the establishment of a more con- 

 venient numerical relation between the two, than could be other- 

 wise secured without far more elaborate contrivances. More- 

 over, if, for the sake of saving room in packing, it be desired to 

 reduce the length of the body, the draw-tube afibrds 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 performance of the instrument when 

 mounted for use. 



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

 that the full value of the draw-tube, and the advantage of giving 

 to it a rack-and-pinion movement of its own (§ 35), 

 come 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. 32. Its eflect is (like 

 the corresponding erector of the Telescope), to an- 

 tagonize the reversion of the image formed by the 

 object-glass, by producing a second reversion, so as 

 to make the image presented to the eye correspond 

 in position with the object. The passage of the rays 

 through two additional lenses, of course occasions a 

 certain loss of light by reflection 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 (§ 35), since these seldom require a D'^'>^-i"1'« ""='1 



,. ,J^, ^ T j2 • -i-> ,1 ^ •;• wi»hEreclor. 



very high degree or 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, without any change in the objective. 

 For when the draw-tube, with the erector fitted to it, is com- 

 pletely pushed in, the acting length of the body (so to speak) is 

 so greatly reduced by the formation of the first image much 

 nearer the objective, that, if a lens of 8-lOths of an inch focus be 

 employed, an object of the diameter of 1| inch can be taken in. 



