DRAW-TUBE AND ERECTOR. 59 



the object (conjugate foci), and the result is a considerable 

 increase in amplification. It must not be forgotten, how- 

 ever, that a limit will soon be found to this method of 

 increasing the magnifying power. The longer the tube, the 

 more of the periphery of the field will be cut off; and 

 further, the aberrations, which have been fairly corrected 

 for a moderate or short tube, may no longer obtain cor- 

 rection when the body of the microscope is unduly 

 lengthened. 



Some of these tubes are plain, while others are divided 

 into inches and parts, so that results once obtained can be 

 recorded, if necessary. 



There are many uses for the draw-tube, which the 

 student will be sure to find out for himself, and he will find 

 that if it is graduated, as shown in the figure, his work will 

 often be rendered much more easy. Some of 

 its uses will be found described under the 

 subjects of Dissecting and Micrometry. 



Into the lower end of the draw-tube the erector 

 (Fig. 33) is made to screw. It consists of a tube 

 about 3 inches long, having a meniscus (concavo- 

 convex) at one end and a plano-convex at the 

 other, a diaphragm being placed about midway 

 between them. The convex side of each lens is 

 turned towards the eye-piece, this combination FlG 

 producing a second inversion of the image, so 

 that it is seen in its natural position. This is of great use 

 to the tyro dissector, as he has then but little difficulty in 

 the use of his dissecting instruments; but if any one 

 who has learned to dissect without it should attempt to 

 use the erector, he will find it is quite as hard to unlearn as 

 to learn. 



The erector, when screwed into the lower end of the 

 draw-tube, enables the observer to employ a greater range 



