OONSTKUCTION OB^ LARGE TELESCOPE LENSES. 165 



Contrary to what niight perhaps be expected, this process is accom- 

 plished with little noise bejond a slight crackling and rustling sound. 

 This is the culminating point in the whole process of glass making, 

 and gives rise not only among unaccustomed onlookers, but also 

 among the skilled workers themselves to mingled feelings of great 

 anxiet}^ and exalted admiration. 



The mold with its fiery contents is then covered wnth an iron 

 plate and pushed over to the cooling oven, which has in the mean- 

 while been carefully heated and opened ready to receive the charge. 

 Here the mold is lifted by a tackle and thrust into the cooling oven, 

 where, after the walls have l)een sealed uj) as tightly as possible, it 

 remains from four to six wrecks undisturbed. Very gradual lowering 

 of the temperature is required, else the cooled mass might l)urst with 

 the slightest touch, or at least show prejudicial strains in the interior. 



When at length the oven is opened the mold is foinul to contain 

 a solid, feebly lustrous, milk-white plate, which is easily removed 

 from its iron bed. 



There now begins a week-long process of grinding and polishing 

 of the glass plate ])reparatory to a preliminary examination as to its 

 freedom from strie, bubbles, and conditions of interior strain. Ex- 

 perience shows that in general only a part of such a plate is of optical 

 A^alue. This part is cut out by means of a glass saw and again 

 heated till soft in a crucilile, which corresptmds approximately with 

 the final form of the ol)jective. After this comes a second gradual 

 cooling during a period of several weeks and another rough polish- 

 ing and testing of the quality of the resulting plate of glass. 



In favorable cases the i)roduct is now^ ready for removal to the 

 optical shop, but conunonly there are ten or more unfavoral)le trials 

 before securing a successful result in the manufacture of a disk of 

 glass for a lens of 1 meter diameter. vSince, as we know% there are two 

 such disks of equal size recjuired for a telescope objective, weeks and 

 months of further work are recjuired for the production of the second. 

 The process is in all respects the same, except that somewhat diti'erent 

 materials are employed for the mixture, corresponding to the differ- 

 ences in optical properties desired. In outward appearance crown 

 and flint lenses do not differ much, but one is somewhat heavier than 

 the other. 



The description just given relates to the most modern methods of 

 glass making as they would now be pursued at Jeiui in the manufac- 

 ture of glass disks for a telescope of 1.^25 meters aperture. In the 

 older processes it was customary to melt a charge about three times 

 as large as required, and after this had reached the i)roi)er color and 

 consistency, to allow the melting oven to cool slowly and thus to take 

 the place of the special cooling oven. On opening the oven the glass 

 block would be found broken in several pieces, and if there was 



