THE 2sATI0NAL PHYSICAL LABORATORY. 



133 



Reichsanstalt, Division I., Main Building, 



ties of solid amorphous fluxes 

 and their chemical constitu- 

 tion. Wlien they began their 

 work some six elements only 

 entered into the composition 

 of glass. By 1888 it had 

 been found possible to com- 

 bine with these in quantities 

 up to about 10 per cent, 

 twenty-eight different ele- 

 ments, and the effect of each 

 of these on the refractive in- 

 dex and dispersion had been 



measured. Tims for I'xamplc the investigators found that by the addi- 

 tion of boron the ratio of 

 the length of the blue end of 

 the spectrum to that of the 

 red was increased; the addi- 

 tion of fluorine, potassium 

 or sodium produced the 

 opposite result. Now in an 

 ordinary achromatic lens of 

 crown and flint, if the total 

 dispersion for the two be 

 the same, then for the flint 

 glass the dispersion of the 

 blue end is greater; that of 

 the red less than for the 

 croM'n; thus the image is not white, a secondary spectrum is tlie result. 

 Abbe showed, as Stokes 

 and Harcourt had 

 shown earlier, that by 

 combining a large pro- 

 portion of boron with 

 the flint its dispersion 

 was made more nearly 

 the same as that of the 

 crown, while by re- 

 placing the silicates 

 in the crown glass by 

 phosphates a still bet- 

 ter result was ob- 

 tained, and by the use 



of three o'Hs'^eS three Reichsanstalt, Bl-ildini; rou Lap.ge Cuhrents and 



'^ ' '" Machinery. 



Reichsanstai.t, Division II., Main Buildini:. 



