Arizona republican., 1918-06-26
LONG JOB MAKING
 OF OPTICAL GLASS
Detroit Free Press)

A visit to modern foundry where castings are made for telescope ob- jectives is almost as impressive as trip through a steel mill. The glass for each component of lens combina tion must possess certain defipite op- tical properties and the materials chosen for that kind of glass must be of the purest quality.
 Before the raw materials are melted together the crucible or pot which is to contain them must be heated gradually to red heat. This part ow the work is drawn out by means of an iron fork run on wheels and it put into position | in the furnace with as little variation i in temperature as possible. Were the fireclay crucible suddenly placed in the | heated furnace it would rapidly shrink and crack into many pieces. After it has been heated and installed into the furnace pieces of broken glass of the same kind are put in the melting pot to form a glaze around its walls. thus protecting the other materials from a violent chemical reaction that would surely follow their sudden contact with
 the fire-cfay that now has attained degree of heat about that of the melt l ing pot.
 One of the chief obstacles in the melting of the glassm fixture is the formation of bubbles. One manner of eXpelling them is to gradually raise the l temperature of the furnace until a maximum degree is attained. In some
 kinds of glass the ebulition of new 1 bubbles continually occurs to such an
 extent that it is practically impossible
 to get rid of numbers of the smallest.
 That is why some of the finest camera
 lenses contain very small bubbles
 xsrhi iph tod 6nfa hot
 must be tolerated in order to possess d
 lens of the finest optical performance.
 All the harm done by minute bubbles
 is to cut off a small per cent of the
 light when the lens is judiciously used.
 After time the liquid glass is tested for bubbles. One way that this is done
 is to insert a pipe into the crucible and
 upon it's withdraway to blow a spher- ical flask of the glass. Examination
 of the flask will disclose the per cent
 of the bubbles present and their ap- , proximate size If they are found to be too large or too numerous the glass is heated longer.
 If the glass is reasonably free from
 the bubbles the melting pot is taker
 out of the furnace and as rapidly placed in kiln where very exact tenn Iperature regulations are possible. The
 
 homogeneous mass. The presence o veins or striae in the material is ac- !, tually worse defect than the presenc, of very small bubbles. Glass bend! 5 under it's own weight and a lens whici contained even an almost imperceptibl n vein would not perform well whei s mounted. The rays of light must pas l | through it without interruption or de d flection from their calculated cours e and unless the material is perfectly ho r mogeneous this is impossible. t A fire clay cylinder is inserted b 2 a mechanical arrangement so it can b d rotated in the pot of molten materia q After stirring until it becomes stiff t viscous mass. in which the cylinder i l. frequently left cooled to a certain de
 gree of temperature. If it were sud s denly cooled further than that limit
 would quickly crack into thousands c