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SUMMARY OF CURRENT RESEARCHES RELATING TO 
I have Lad the collar, in which the draw-tube slides, screwed to the main 
body-tube (fig. 18). The screw is placed below, while a shoulder is 
placed above, where the screw usually is. This is an important point, 
because it makes a sound and strong slide, which will not become shaky 
as is often the case. This kind of fitting is used in the best telescopes. 
In connection with this I have an ingenious adaptation of Messrs. 
Watson to show you for a mechanical draw-tube. 
There are two kinds of mechanical draw-tubes at present in use. 
The first fitted to a Microscope was that by Powell, who cut the inside 
Fig. 18. Fig. 19. 
tube and placed the rack in the cut (fig. 19). This plan is quite feasible 
in his Microscope on account of the large size of the tubing; but when 
we come to small tubing such as this, it is hardly practicable. For this 
reason No. 2 w r as invented. It has the inner tube intact, and the outer 
tube cut, and a box placed over the cut in the outer tube (fig. 20). This 
is expensive to make. Messrs. Watsons’ plan consists in cutting neither 
tube, but in making the outside tube large enough to take the rack, 
which projects from the inner tube, and then cutting the collar to allow 
it to pass (fig. 21). This is the cheapest of the three, and at the same 
