_a._ By analysis:--
Silica, 44·30%; oxide of lead, 43·05%; potassa, 11·75%. This is Guinand's
'dense optical glass.'
_b._ Materials used:--
1. Purest quartz, 42%; red lead (finest), 42%; purified potash, 14-3/4%;
purified nitre, 1-1/4%. These are the proportions used for the last.
2. (Korner.) Finest quartz (reduced to powder, treated with hydrochloric
acid, washed, and dried), 47-1/2%; red lead, 38-1/4%; cream of tartar,
14-1/2%. The above are used by opticians in the construction of achromatic
object-glasses.