ACCESSORY INSTRUMENTS. 



131 



quently closed with sealing-wax, if it be required to preserve 

 the fluid for future examination. 



Mr. Varley, in the year 1831, greatly improved this form 

 of instrument, and gave to it the name of capillary tablet or 

 cage, in a paper published in the forty-eighth volume of the 

 Transactions of the Society of Arts. This great improvement 

 consists in making a channel aU round the object-plate, so that 

 the fluid and the animalcules in it are retained at the top of 

 the object-plate only, by capillary attraction, and will bear turn- 

 ing about in all directions without leaving the top, provided it 

 be not suddenly shaken. The cover also is made to screw 

 down upon the object-plate, and not to slide as in the pre- 

 viously described instrument; but in practice it has been 

 found most convenient to adopt the sliding tube, as the act of 

 screwing sometimes deranges the objects. The ^ late of brass 

 to which the tube supporting the tablet and cover is attached, 

 is of a circular figure, slightly flattened on two opposite 

 sides, for convenience of package, as several of them can be 

 contained in a small cylindrical case. The improvement made 

 by Mr. Varley, in the object-plate or tablet, is now adopted 

 by all our first-rate microscope makers, but with some few 

 sight modifications ; one of these instruments, as constructed 

 at the present time, is represented by fig. 82 in elevation, and 



in section by fig. 83. 



Fig. 82. 



Fig. 83. 

 glass, a, is cemented. 



A B in both figures ex- 

 hibit the flat plate of 

 brass to which the short 

 tube, carrying the object- 

 plate, or tablet, is fixed; 

 d, fig. 83, exhibits the 

 piece of brass into which 

 the tablet, c, is fastened, 

 b the tubular part of the 

 cover, into the rim of 

 which the thin plate of 



This thin glass cover is often either 

 broken or becomes uncemented ; to remedy the inconvenience 

 of re-cementing, Mr. Powell adopts a very excellent plan, by 

 9* 



