12 



Microscopical Essays. 



other: the heated glafs following each rod, will be finer, in 

 proportion to the length it is drawn to and the fmallnefs of the 

 rod ; in this manner you may procure threads of glafs of any de- 

 gree of finenefs. Dired the flame to the middle of the thread, 

 and it will be inftantly divided into two parts. When one of 

 the threads is perfectly cool, place it at the extremity of the 

 flame, by which it will be rendered round ; and if the thread of 

 glafs be very fine, an exceeding fmall globule will be formed. 

 This thread may now be broke off from the rod, and a new one 

 may be again drawn out as before, by the affiftance of the other 

 glafs rod. 



The fmall ball is now to be feparated from the thread of glaft; 

 this is eafily effe&ed by the (harp edge of a piece of flint. The 

 ball mould be placed in a groove of paper, and another piece of 

 paper be held over it, to prevent the ball from flying about and 

 being loft. A quantity of globules ought to be prepared in this 

 manner ; they are then to be cleaned, and afterwards placed in 

 the cavities of the tripoli, by means of a delicate pair of nippers. 

 The globules are now to be melted a fecond time, in order to 

 render them completely fpherical ; for this purpofe, bring one of 

 the cavities near the extremity of the flame, directing this towards 

 the tripoli, which muff be firfl heated ; the cavity is then to be 

 lowered, fo that the flame may touch the glafs, which, when it is 

 red hot, will affume a perfect globular form ; it muft then be re- 

 moved from the flame, and laid by; when cold, it mould be 

 cleaned, by rubbing between two pieces of white paper. Let it 

 now be fet in a brafs cap, to try whether the figure be perfect. If 

 the objea is not well defined, the globule muft be thrown away. 

 Though, if it be large, it may be expofed two or three times to 



