222 
Freedom from seeds or bubbles is 
desirable for the sake of appearance, 
but it is not necessary except in reticles 
or windows and other elements in a 
tocal plane. Appearance, therefore, 
poses quite a problem for the glass- 
maker because the barium glasses so 
necessary in photographic objectives 
today are especially difficult to make 
seed-free. 
As can be seen from the foregoing, 
the production of glass for the ordi- 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
Europe, the most famous being the 
40-inch refractor of the Yerkes Observ- 
atory at Williams Bay, Wis., the larger 
telescopes are all of the reflecting type. 
The manufacture of a large lens 
necessitates a single large chunk of 
perfect optical glass which must then 
be brought to the required shape by 
heating and softening it in a ceramic 
mold of proper size. Such large 
chunks of glass are extremely difficult 
to obtain, usually requiring many 
40 33 30 23 
af fone AT 
CI] alee eal aaleaa| te 7 lee 
ASISEIS_|ZLL 7: 
< 
VY vatue ———+ 
Ficure 1.—Chart showing relation of index and reciprocal dispersion of the common types 
of glass. Most glasses lie near a curve in which dispersion increases with index; glasses far 
off this curve are especially useful to the designers of optical systems. Diagram courtesy 
Bausch & Lomb Optical Co. 
nary types of optical instruments is 
fraught with many difficulties. And 
when the glassmaker is called upon to 
produce lenses or mirrors for astro- 
nomical telescopes, his problems are 
increased manyfold. It will be the 
purpose of the remainder of this paper 
to show how these difficulties were 
overcome, first at the National Bureau 
of Standards and, later, at the Corning 
Glass Works. 
Although there have been some 
notable large lens blanks made in 
melts of glass) Upon examination of 
the resultant lens, it may be found to 
be unsatisfactory, and all the work has 
to be done again. Nor are the diffi- 
culties over even then. To complete 
the lens requires the accurate grinding 
and polishing of two faces. For a 
mirror for a reflecting telescope, only 
one face of the disk has to be ground 
and polished. Also, the glass for the 
disk may contain considerable seed 
and striae unless they are on the sur- 
face and interfere with the figuring 
