776 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
glass of sufficient size and homogeneity for complete spectroscopic 
measurement could not be obtained in this way. Nevertheless, these 
small pieces afforded proof that the above acids yielded glasses of extra- 
ordinary variety, and thus showed that considerable gradations in 
refractive index and dispersion could be obtained by their use. 
The next advance was to prepare larger samples of glass. For this 
purpose a Fletcher gas furnace was found to be of the greatest service. 
With this furnace a rise in temperature up to the melting-point of 
metallic nickel could be obtained in about a quarter of an hour. The 
furnace consists of the usual heating chamber made of firebrick, and, in 
addition, of a so-called injector burner. The latter is formed of an iron 
tube, with the end towards the furnace covered with wire gauze, and the 
other pierced by a twyer through which the compressed air is forced. 
A pressure of air of 4-8 cm. of mercury is sufficient to effect the com- 
plete combustion of the gas admitted through a side-tube. The wire 
gauze at the end of the tube prevents the flame from passing backwards 
and causing an explosion of the combustible mixture in the tube. With 
this apparatus samples of glass of 150 grm. weight, and, later, up to 
10 and 25 kgrm., were obtained, and they were found to satisfy all 
requirements, not only for scientific, but also for practical purposes. 
By these means the optical characters of a large number of metallic 
oxides and acids were soon recognized, and it was found possible to 
incorporate in the glass, in amounts above 10 per cent., twenty-eight 
other bodies besides the five hitherto used. These were : — 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. 
The first object of the investigation — the power of obtaining grada- 
tions in refraction and dispersion — was soon attained. For the removal 
of the secondary spectrum, however, only comparatively few elements 
were found to offer suitable variations from the ordinary course of the 
dispersion. Of these boric acid produced a contraction of the blue and 
widening of the red end of the spectrum, while fluorine, potassium, and 
sodium had an opposite effect. For all other elements the course of the 
dispersion was the same as with silicate glass. Accordingly, since the 
ordinary flint glass shows an extension of the blue end of the spectrum 
as compared with crown glass, as high a percentage of boric acid as 
possible must be incorporated with it in order to obtain a perfect 
compensating effect. Thus boric acid has become the fundamental 
constituent of all flint glasses destined to diminish the secondary 
spectrum. The conditions are less favourable when it is desired to 
effect the compensation by widening the blue end of the spectrum of a 
crown glass. Of the three elements — fluorine, potassium, and sodium — 
suitable for this purpose, the last can only be introduced into silicate 
glass in very small quantities, and the potassium in amounts not exceed- 
ing 25-30 per cent. ; this is because of their hygroscopic effect. 
The chief characteristic of phosphoric acid — that of yielding glasses 
of comparatively slight dispersion with high refractive index — was made 
use of to solve the problem ; for it had been found that, with equal ratio 
of refraction and dispersion, glasses of higher refractive index showed 
