348 PHYSICAL LABORATORY OF GREAT BRITAIN. 



glasses having a different relation Itetween dispersion and refractive 

 index than that found in the material at the disposal of opticians. Stokes 

 and Harcourt had already made attempts in this direction, but with no 

 marked success. In 1881 Abbe and Schott at Jena started their work. 

 Their undertaking, the}' write five 3'ears later in the first catalogue of 

 their factory, arose out of a scientific investigation into the connection 

 between the optical properties of solid amorphous tluxes and their chem- 

 ical constitution. When they began their work some 6 elements only 

 entered into the composition of glass. By 1888 it had been found possible 

 to combine with these in quantities up to about 10 per cent 28 different 

 elements, and the effect of each of these on the refractive index and 

 dispersion had been measured. Thus, for example, the investigators 

 found that b}' the addition of boron the ratio of the length of the 

 blue end of the spectrum to that of the red was increased; the addition 

 of fluorine, potassium, or sodium produced the opposite result. Now 

 in an ordinary achromatic lens of crown and flint, if the total disper- 

 sion 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 crown; thus 

 the image is not white, a secondary spectrum is the result. Abbe 

 showed, as Stokes and Harcourt had shown earlier, that by combining 

 a large proportion of boron with the flint its dispersion was made 

 more nearly the same as that of the crown, while by replacing the 

 silicates in the crown glass by phospates a still better result was 

 obtained, and by the use of three glasses three lines of the spectrum 

 could be combined. The spectrum outstanding was a tertiary one and 

 much less marked than that due to the original crown and flint glass. 

 The modern microscope became possible. 



The conditions to be satisfied in a photographic lens differ from 

 those required for a microscope. Von Seidel had shown that with the 

 ordinary flint and crown glasses the conditions for achromatism and 

 for flatness of field can not be simultaneousl}^ satisfied. To do this we 

 need a glass of high refractive index and low dispersive power or vice 

 versa; in ordinary glasses these two properties rise and fall together. 

 Thus ci'own glass has a refractive index of 1.518 and a dispersive 

 power of 0.0166, while for flint the figures are 1.717 and 0.0339. By 

 introducing barium into the crown glass a change is produced in this 

 respect. For barium crown the refractive index is greater and the 

 dispersive power less than for soft crown. With two such glasses, 

 then, the field can be achromatic and flat. The wonderful results 

 obtained by Dallmeyer and Ross in this country, b}' Zeiss and Stein- 

 heil in Germany, are due to the use of new glasses. They have also 

 been applied with marked success to the manufacture of the object 

 glasses of large telescopes. 



But the Jena glasses have other uses besides optical. "About 

 twenty years ago" — the quotation is from the catalogue of the German 



