TURN-TABLES FINISHING 



451 



bands ; which plan is extremely convenient not merely for the 

 saving of space, but also for preserving the objects from dust. Should 

 any more special protection be required, a thin glass cover may be 

 laid over the top of the cell, and secured there either by a rim of 

 gum or by a perforated paper cover attached to the slide; and if 

 it should be desired to pack these covered slides together, it is only 

 necessary to interpose guards of card somewhat thicker than the 

 glass covers. 



Turn-table. This simple instrument (fig. 380), devised by 

 Mr. Shadbolt, is almost indispensable to the rnicmscopist who desires 

 to preserve prepara- 

 tions that arc 

 mounted in any 

 medium ' beneath 

 circular covers ; since 

 it not only serves 

 for the making of 

 those 'cement-cells' Fi<;. ?>m>. Shadbolt's turn-table. 

 in which thin trans- 

 parent objects can Itc best mounted in any kind of medium.' but 

 also enables him to apply his varnish for the securing of circular 

 cover-glasses not only with greater neatness and quickness, but also 

 with greater certainty than he can by the hand alone. The only 

 special precaution to be observed in the use of this instrument is 

 that the cover-glass, not the slide, should be ' centred ;' which can 

 be readily done, if sei'p-i'<d concentric circles have been turned on the 

 rotating-table. by making the cover-glass correspond with the one 

 having its own diameter. A num- 

 ber of ingenious modifications have 

 been devised in this simple instru- 

 ment with a view to securing exact 

 centring. The most practicable 

 and inexpensive of these is an 

 application of Mr. E. H. fSrittith's 

 device shown in its improved form 

 in fig. 381. 



The centre of the table marked 

 with circles has a straight spring 

 attached to it beneath. The slide, 

 being placed between the two pins 

 A and B in this centre, is partial 1\ 

 rotated against the spring and 

 pushed forward, when the spring 

 keeps it between the two pins and a third fixed pin, 1), at the upper 

 side of the slide, centring it perfectly for width. The fourth pin, 

 E, at the left end, 1^ in. from the centre, is for length, and allows 

 the slide to be always placed in the same relative position. The 

 recent improvements add much to the value of the table. One of 

 them is a countersunk decentring wheel and pin, C, which may be 

 seen at the upper right-hand side of the slide. The axle of the 

 wheel passes through the table and is furnished underneath with a 



(r G 2 



Fit;. 381. Griffith's lum-t;iI>lH. 



