162 



SELECTION AND TJSE 



In the turn-table just described, provision is made to obviate 

 this difficulty. A heavy-headed screw, of the precise weight 

 necessary, is screwed into the under surface of the table, and 

 gives a perfect balance to the wheel. It then runs smoothly 

 and truly. 



Numerous attempts have been made to produce a self-center- 

 ing table, i. e. , one in which the 

 slides would be truly centered 

 without requiring care and skill 

 on the part of the operator. One 

 of the earliest forms was that of 

 Dr. Matthews, the centering part 

 of which is shown in Fig. 36. 

 Upon the surface of the table he 

 arranges two triangular plates 

 of brass, which rotate upon pins 

 placed at equal distances on each 

 side of the centre, and as the 

 plates are of the same size, when- 

 Fig. 36. MATTHEW'S TURN-TABLE, ever their inner faces are par- 

 allel, these faces must be equi- 

 distant from the centre. Hence, when a slide with parallel 

 sides is placed between them, and the plates turned so as to 

 press upon the sides of the slide, the slide will be truly cen- 

 tered so far as its width is concerned. It is centered for length 

 by a stationary pin, against which the end is always brought. 

 Slides of irregular size are therefore centered only one way. 



There are at present, however, before the public, two tables 

 which centre slides accurately in both directions. One was in- 

 vented by C. Mason Kinne, of San Francisco, who describes it 

 as follows: "As will be seen from the engravings, Figs. 37 and 

 38, which are reduced one-half, the slide will be grasped autom- 

 atically, upon removing the finger from the lever, the spiral 

 spring causing the clutches to instantly clasp the slide, and 

 retain it in a central position. One corner of either end of the 

 slide projects sufficiently for the purpose of taking hold with 

 one hand, while the other is pressing the lever, and can be 

 fixed or removed without pushing along a circular disc to its 

 edge. The slots are made to allow movement enough, so that 



