ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
775 
tlie crown and flint glass now at our disposal, the dispersion always goes 
hand in hand with the mean refractive index, so that the higher dis- 
persion always belongs to the higher index, and vice versa. The aber- 
ration would be completely or at least approximately compensated if we 
possessed materials in which a relatively low refractive index was com- 
bined with a . high dispersion, or a high refractive index with a relatively 
slight dispersion. It would then be possible, by a suitable combination 
of such a material with the ordinary crown and flint glass, to remove the 
chromatic and spherical aberration independently of one another, and 
thus to fulfil the essential condition on which the removal of the 
chromatic difference depends.” 
The perfecting of delicate optical instruments appeared, therefore, to 
depend on an advance in the art of glass-smelting, so that it should be 
possible to produce glasses suitable for the removal of the secondary 
spectrum, in which dispersion and mean refractive index would be 
differently related than in the glasses then used. 
The uniformity in optical properties which characterizes most kinds 
of glass is due to the small number of materials used in their manufac- 
ture, viz. silica, alkalies, limestone, lead, and, in lesser degree, clay and 
thallium. The great aim of the authors was to advance beyond these 
narrow limits, and, directed by a methodical study of the optical charac- 
ters of various chemical elements, to make use of such as appeared 
suitable to the end they had in view. They were in their work chiefly 
guided by the following considerations : — 
(1) The composition of the glass must be regulated so that it shall 
not have too strong an action on the walls of the containing vessel, and 
thus lead to the introduction of foreign matter. 
(2) It must be kept homogeneous by energetic stirring while in the 
melted state, so that it may be free from striae of different refractive 
indices to that of the main mass. 
(3) It must be kept free from cloudiness, crystal formation, and 
blisters during the processes of melting and cooling. 
(4) It must bear reheating up to the melting-point without cloudi- 
ness or crystal separation. 
(5) It must be kept free from strain by a suitable process of 
cooling. 
(6) It must offer sufficient resistance to atmospheric agencies, and 
especially must show no signs of hygroscopic properties. 
(7) It must be colourless. 
(8) It must have sufficient hardness to allow the grinding, polishing, 
and shaping of the refractive surfaces. 
Only relatively few inorganic anhydrides give rise to amorphous 
bodies when fused with certain metallic oxides and cooled. Hitherto 
silica was the only one which answered the above requirements, but 
boric, phosphoric, and arsenic acids were known to give glassy bodies in 
certain of their compounds. The authors accordingly first turned their 
attention to phosphoric and boric acids, and combined them with a great 
number of metallic oxides, in order to discover a combination without 
hygroscopic properties. The fusions were conducted in quite small 
porcelain crucibles of 20 to 30 ccm. contents with an ordinary laboratory 
blow-pipe. In spite of constant stirring during the fusion, pieces of 
