261 



How ro Make Thin Cover Glass. 



By G. J. Burgh, Esq. 



(Read April 2Uh, 1874.; 



Take a piece of glass tube of about :^in. bore, seal up the end 

 with the blow-pipe, and continue the heat until the glass is so soft 

 that it will fall out of shape, unless you keep turning it round ; 

 remove it from the flame, and blow into it with all your strength. 

 It will be seen to swell, at first slowly, and then suddenly to a 

 large bubble of very thin glass. Supposing the tube to have been 

 sealed up with as little glass as possible, it may be blown out to 

 about 4 inches diameter. When cold, break it up, and cut the 

 pieces to shape with a " writing diamond." The glass in this state 

 is of course convexo-concave ; practically this is of little conse- 

 quence unless the objects are to be mounted dry, when it is liable 

 to be broken. In order to flatten it, place a piece of the thin glass 

 on a perfectly flat piece of platinum foil, and depress it for a 

 moment into the Bunsen flame ; as soon as it is red hot, it will 

 sink down to the flat foil. This also has the effect of annealing it. 



On measuring a piece of this glass with the micrometer, I found 

 it to be= ^J^Q^inch = '0004 inch. 



In the " Monthly Microscopical Journal," vol. viii., page 270, 

 Dr. Royston Pigott says : — " The thinnest glass in my possession 

 measures 2i- thousandths." Now t\ thousandths = "0022, and 

 '.%%\\ = 5-5. So that his thinnest glass is 5 J times the thickness 

 of mine. 



I enclose a small piece, the thickness of which is about -0004 in., 

 perhaps less. I do not advise anyone to make all his covers this 

 way, but only the very thinnest. 



