THE MICROSCOPE AND ITS PARTS. 37 



tus that is a boon to any one desiring to do a little ama- 

 teur glass-blowing. A strip of tin about six indies long 

 and two wide is rolled, without solder or fastening of 

 any kind, into a tube about half an inch in diameter, 

 after two holes, each about one-fourth inch in diameter, 

 have been punched so that they shall be on opposite 

 sides of the tube, and high enough to be a short dis- 

 tance above the tip of the gas-burner. This simple ar- 

 rangement is forced over an ordinary burner, so that the 

 holes are just above the tip, the spring of the tin hold- 

 ing it in place; the gas is lighted at the upper end, 

 where it burns without smoke and gives a strong heat, 

 the flame being easily regulated, and, with ordinary care, 

 not flashing into the tube. It is entirely successful. 



Evaporation of the water will take place from be- 

 neath the thin cover, sometimes quite rapidly, and the 

 observer will at first be surprised at the way in which 

 his objects will be swept out of the field before an ad- 

 vancing wave that leaves the glass nearly dry behind it. 

 The water in the cell is drying up, and a fresh supply 

 must be added if the objects are not to be entirely lost. 

 Here is another advantage in using square covers on cir- 

 cular cells. The four corners project beyond the cement 

 ring, and by applying the camel's-hair brush, wet with 

 water, to the slide beneath any one of these projections, 

 the drop will run in and fill the cell by capillary attrac- 

 tion. This supply is much more easily added than if 

 circular covers are used, and after a little experience the 

 fresh drops can be applied while the eye is at the eye- 



