164 CONSTRTTCTION OF LAROE TP^LESOOPP: LPINSES. 



To l)('i»iii tlu' process of construct ion a crucible of fire-proof clay, 

 which aliviuly 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 i^eehole about as large as a man's head in the wall of the 

 oven. 



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

 beginning of the year 18S0 only two kinds of optical glass were in use, 

 of which one — the so-called crown glass — wascomposed 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 o])tical glass produced b}- the intermixture 

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

 barium, antimony, which are distinguished by difi'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 whether it is to be emplo3'ed for visual or photo- 

 graphic observations. 



About thirty hours is consumed in the introduction of tlie ma- 

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

 1,G00° to l,yOO° tlie melting i)ot cracks or even tlie stones of the oven 

 burst — the im])urities are then skimmed olf from the surface, and 

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

 shaped, white-hot clay cylinder. 



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

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

 destroy the homogeneitv reipiired for optical purposes. This dif- 

 ficult}^ is chiefly overcome by the continuous stirring of the melted 

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

 charge assumes the pi-o})er color an<l degree of fluidity. When this 

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

 wheeled ti'uck with long handles is backed up to tht» opening. T^vo 

 l)rojecting [)ieces of the truck I'each out under a ring which is made 

 for this ])urj)ose on the clay melting })ot, and the latter is carefully 

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

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

 this is an operation involving a gi'eat risk that the fragile white-hot 

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

 from the oven. 



The melting })ot is next moved over to a great circular iron mold, 

 and is then set down upon the lloor, in order to reenforce the pol with 

 an iron band. On opposite sides of the iron band are steel pins 

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

 ment the pot is lifted above the mold and its contents poured therein. 



