164 CONSTRUCTION OF LARGE TELESCOPE LENSES. 



To begin the process of constniction a crucible of fire-proof clay, 

 which already has been warmed gradually for several days, is placed 

 within a melting oven of peculiar construction. This oven is then 

 closed and slowly heated to white heat, while at the same time the 

 materials to compose the glass are admitted to the melting pot 

 through a peehole about as large as a man's head in the wall of the 

 oven. 



The material varies with the kind of glass to be nuide. Until the 

 l)eginning of the year 18S0 only two kinds of optical glass were in use, 

 of which one — the so-called crown glass— was composed of quartz sand, 

 potash, soda, and calcspar, and the other — the so-called flint glass — 

 was composed of quartz sand, potash, and lead oxide. There are now 

 more than 100 vareties of optical glass produced by the intermixture 

 of other materials, such as phosphorus, boric acid, magnesium, zinc, 

 barium, antimony, which are distinguished by ditt'erent properties 

 of dispersion and refraction. The choice of the proper, glass for the 

 two lenses depends on the purpose which the telescope is to serve, 

 and particularly Avhether it is to be employed for visual or photo- 

 graphic observations. 



About thirty hours is consumed in the introduction of t\K- ma- 

 tirals. If there be no mishap — for sometimes at the temprature of 

 1,600^ to 1,800° the melting jjot cracks or even the stones of the oven 

 burst — the impurities are then skimmed off from the surface, and 

 for about fifteen hours the mixture is stirred by means of a hook- 

 shaped, white-hot clay cylinder. 



AVhen the ingredients of the glass are melted they have a tendency 

 to separate in layers according to their specific gravities, and thus to 

 destroy the homogeneity required for optical purposes. This dif- 

 ficulty is chiefly overcome by the continuous stirring of the melted 

 mixture. By repeated tests the moment is at length found when the 

 charge assumes the proper color and degree of fluidity. Wlien this 

 moment arrives the farther side of the oven is opened and a two- 

 wheeled truck with long handles is backed up to the opening. Two 

 projecting j^ieces of the truck reach out under a ring which is made 

 for this purpose on the cla}^ melting pot, and the latter is carefully 

 lifted from the floor of the oven. On account of the overflow of 

 melted glass, wdiich often cements the pot to the bottom of the oven, 

 this is an ()})eration involving a great risk that the fragile white-hot 

 clay melting pot may break, owing to the shaking re(iuired to free it 

 from the oven. 



The melting pot is next moved over to a gi-cat circular iron mold, 

 and is then set down upon tlic lloor, in order (o ivenforce the po( with 

 an iron band. On opposite sides of the iron band are steel [)ins 

 fitting on hooks attached to the tiMick. By means of this arrange- 

 ment the pt)t is lifted above the mold and its contents poured therein. 



