no THE NEW WORLD OF SCIENCE 



The skill required for its successful production is quite properly 

 comparable to that required for exact quantitative analysis. 

 There was no time to look into the many scientific problems 

 encountered. It was the same insistent demand for production, 

 and still more production, but always within the limits as 

 regards quality, which could not be greatly extended even in 

 the war emergency. Good optical glass must be homogeneous, 

 both chemically and physically, it must have definite refractive 

 indices for different wave lengths of light, it must be as free 

 as possible from color, have a high degree of transparency, 

 extreme stability against weather and reagents, and have tough- 

 ness as well as hardness. 



When glass is chemically homogeneous it is free from striae, 

 bubbles, stones, crystals, and cloud. Striae are variously known 

 as veins, cords, threads, and ream by glass-makers. Striae 

 come from a variety of sources and even when the melt is 

 comparatively free from them, lines of flow may be set up by 

 suddenly moving or jarring the pot. These lines of flow may 

 bring in glass from the sides, where it has been affected by the 

 pot, from the bottom, where there may have been selective 

 settling, or from the top where the volatilization may have 

 changed the composition enough to make the extremely small 

 difference in refraction which spells striae. Striae are fre- 

 quently due to excessive action of the glass upon the pot, insuffi- 

 cient or inadequate stirring, incorrect temperatures, or cooling 

 of the pot in arches, where the heat is so great as to cause the 

 stiff crust on the partly cooled pot to re-melt and start lines of 

 flow by convection. Striae usually show a high percentage 

 of silica and alumina. One of the important remedies is proper 

 stirring and this differs according to the glass and other con- 

 siderations, such as the selection of a pot built with due regard 

 for the kind of glass which is to be melted in it. 



For some uses small striae if in one plane, are of such 

 trifling consequence that the glass can be used for certain types 

 of lenses. This led to the American war-time method of roll- 



