442 CONCAVE MIRRORS. 



Melt in a ladle 114? r (6 x 19) of lead and 174e r (6 x 29) of tin, 

 and stir the molten liquid with a splinter of wood. It should not 

 be poured out too hot. To avoid this, let it cool in the ladle after 

 melting, and then heat it again ; pour out when a small portion is 

 still unmelted. The dull grey film which during the melting is 

 formed on the surface of the alloy remains behind in the spoon, if 

 the metal is allowed to run into the mould from the edge of the 

 circular vessel. The latter will, in that case, be filled with pure 

 bright metal. The metal should stand about 2 mm above the edge. 

 As soon as a few small dull spots appear on the bright surface, which 

 indicate that the mass is beginning to solidify, take a piece of stiff 

 paper, draw its edge over the surface so as to remove all particles 

 which have become solid, and to make the surface bright again ; and 

 now immediately press the lens upon it, holding it somewhat inclined, 

 and placing it thus upon the metal. If laid on horizontally, too many 

 air-bubbles will remain between glass and metal. It should also be 

 pressed down very quickly and be immediately lifted up again, or 

 it will crack in consequence of the heating. Immediately after lift- 

 ing up the glass a cracking sound is usually heard ; it is due to 

 the partial separation of the metal from the glass. When quite 

 cold the metal will part from the glass of itself, or a slight help 

 with the finger will make it separate. ' There are always layers of 

 vapour and condensed air upon the glass, both of which cause 

 numerous bubbles to appear on the surface of the metal. The first 

 and second casts are, from this cause, mostly quite useless. These casts 

 are therefore melted again, and the third cast is generally satis- 

 factory, even if a few vesicles should still make their appearance. 

 The whole is now turned out, the glass below and the metal on the 

 top, and upon the latter a piece of sealing-wax, 3 or 4 cm long, is 

 pressed, which melts at the end, and adheres after cooling, so as to 

 serve as a handle for clamping the mirror into the fork of the retort- 

 stand. 



Such thin mirrors are easily bent out of shape, especially when 

 some force is applied in separating the glass from the metal. The 

 glass should, therefore, not be pressed too deeply into the molten 

 mass, while on the other hand care must be taken lest the metal 

 reaches or overflows the edge of the glass. Before proceeding to 

 the actual casting some preparatory practice with a piece of common 

 glass is advisable, in order to obtain the requisite facility in pressing 

 down and raising the glass without cracking it. 



The metallic surface has, immediately after solidification, the exact 

 figure and outline of the glass, but soon afterwards, sometimes even 

 before the glass is completely removed, the surface shows fine 



