roNSTurcTroN of lakok tklkscotk lknsks. 105 



Conlrary to wlini miii'lit ])('rliiii)s be expected, (his ])roeess is aceoni- 

 plislied with litde noise hevoml a slii>-ht crnckiiiiii' and riistlin<2: sound. 

 This is the eidiniiiarnti!,' ])oiiit in (he wliole jji'ocess of i^lass niakin<r, 

 and li'ives rise not only anion<>; nnaeciistonied onlookers, but also 

 anionir the skilled workers themselves to mingled feelinj^s of great 

 anxiety and e\al(ed admiration. 



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

 plate and })ushed over to the cooling oven, which has in the mean- 

 while been carefully heated and oix'iied ready to ivceive (he charge. 

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

 where, after the walls have been sealed u}) as tightly as possible, it 

 remains from four to six weeks undisturbed. Very gradual lowering 

 of the temi)erature is required, else the cooled mass might burst with 

 the slightest touch, or at least show ])rejudicial strains in the interior. 



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

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

 from its iron bed. 



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

 of the glass plate ])re})aratory to a pi'eliminary 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 

 value. This part is cut out by means of a glass saw and again 

 heated till soft in a crucible, which corresponds approximately with 

 the final form of the objective. After this comes a second gradual 

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

 ing and testing of the (juality of the resulting plate of glass. 



In favoral)le cases the product is now ready for removal to the 

 optical shop, but commonly there are ten or more unfavorable trials 

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

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

 such disks of ecpial size required for a telescope objective, weeks and 

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

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

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

 ences in optical properties desired. In outward ap])earance crown 

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

 th(> other. 



The descri])lion just given relates to (lie most modern methods of 

 glass making as they would now be j)ursued at Jena in the manufac- 

 ture of glass disks for a telesco[)e of i.'2i) meters a})ertui-e. In the 

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

 as large as reciuired, and after this had reached the proper color and 

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

 the ])lace of the special cooling oven. On opening the oven the glass 

 block would be found broken in sevei'al pieces, and if there was 



