OPTICAL GLASS FOR WAR NEEDS 105 



a state of arrested development awaiting the perfection of glass 

 which would offer a greater diversity in mean index, and mean 

 dispersion, and render possible a higher degree of achromatism, 

 thus diminishing the secondary spectrum. This plea attracted 

 the attention of Otto Schott, and after communicating with 

 Abbe, the two began an investigation of the problems, and 

 started first of all to determine the chemical-physical principles 

 underlying the making of optical glass. In experimenting with 

 various combinations of elements new to the glass industry, 

 several limitations had to be borne in mind. First, the flux 

 must not act upon the material of the crucible and so absorb 

 impurities. Second, elements which evaporate during the 

 process tend to produce veins and must not be used. Third, 

 cloudiness, crystallization, and bubbles must be avoided in the 

 process of melting, cooling, and subsequent re-heating. Fourth, 

 it must be possible to bring the glass from the plastic to the 

 solid state without producing stress. Fifth, glass must not be 

 tarnishable or hygroscopic. Sixth, it must be colorless and 

 physically strong enough to bear the manipulation necessary in 

 grinding and polishing. 



Beside silicic acid, the only glass-making acids were boric 

 acid and phosphoric acid and perhaps arsenic acid. There was 

 a tradition that these acids only gave tarnishable glass, but 

 experiments showed that phosphoric. and boric acids could be 

 combined with many metallic oxides and in addition to the six 

 usual elements, namely, silicon, potassium, sodium, lead, 

 calcium, and oxygen, the following were introduced by degrees 

 in quantities of at least 10 per cent : boron, phosphorus, lithium, 

 magnesium, zinc, cadmium, barium, strontium, aluminium, 

 beryllium, iron, manganese, cerium, didymium, erbium, silver, 

 mercury, thallium, bismuth, antimony, arsenic, molybdenum, 

 niobium, tungsten, tin, titanium, uranium, and fluorine. 



It was soon seen that by the introduction of new elements 

 the variation of the hitherto fixed relation between refraction 

 and dispersion could be attained. On the other hand, very 

 few of the elements rendered the dispersions of crown and flint 



