180 Mr. Carl Barns on the Fusion 



9. Sight-tubes. — Grunow's excellent cathetometer is placed 

 on a pier, so that the prism of the instrument is only about 

 50 centim. off from the fusion-tube. A tall, hollow screen, 

 30 centim. broad, 70 centim. long, and 1 centim. thick, fed 

 with cold water, and movable on a slide, is interposed 

 between furnace and cathetometer. Slots, cut through the 

 screen and closed by plate glass, correspond to the two posi- 

 tions of the telescope ; and the lines of sight pass through 

 S, S, near the top and the bottom of the furnace (see figs. 1 

 and 2). Thus the ends of the fusion-tube, one of which pro- 

 jects above the clay tube F F, and the other is seen through 

 the perforation in C (figs. 1 and 5), appear as sharp 

 lines in the telescope, against the red-hot background of 

 clay. _ 



It is necessary, however, to prevent the escape of flame and 

 gas at S, and hence these holes are provided with porcelain 

 tubes A (only one shown in the figure) about 15 centim. long, 

 the outer end of which is ground off square and closed with 

 a piece of plate glass b, by aid of a clamping device aa. 



10. Vertical Micrometer. — Figures 3 and 4 give a full 

 account of this instrument (also made by W. Grunow), both 

 of which are sectional elevations at right angles to each other. 

 The millimetre-screw plays easily through the massive block 

 of brass, P P, and the fixed lock-nut g g. P P is bolted down 

 to the rigid bridge of brass, ISTN, by means of screws, RE, 

 and the counterplate U U. The ends of N N are provided 

 with sleeves, p p, and a clamp-screw, M, whereby the whole 

 micrometer may be moved up or down or fixed in any posi- 

 tion along the uprights, Q Q. To secure insulation of the 

 screw K k, R, R are surrounded by jackets n, n of hard rubber, 

 and plates, i, j, q of this material are suitably interposed be- 

 tween P P, N N, U U, and other parts. 



Sufficiently wide slots are cut in the bridge, N N (see 

 figures), whereby the plate may be shifted in any direction, 

 and then clamped in position. 



In view of the heat which rises from the furnace, an N- 

 shaped screen, X X, through which water rapidly circulates 

 (entering and leaving at diagonally opposite points, V and Y, 

 at the top), nearly envelopes the micrometer, The micro- 

 meter-screw passes through a narrow tube in the bottom of 

 XX ; hence the screen must also be adjustable, and a way is 

 shown in the figure. Thus the micrometer and its water- 

 screen slide as a single piece, along the upright Q Q, Q and 

 Y are joined by a sufficient length of rubber hose, a connexion 

 merely indicated in the figure. 





