ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
777 
an extension of the blue end of the spectrum. Thus by the use of 
phosphoric acid for the crown glass and a glass containing a high 
percentage of boric acid for the flint, it was found possible to almost 
completely remove the secondary spectrum. 
While introducing material to produce certain optical effects, an 
important point to be considered was the modification of the composition 
of the glass, so that its external characters should remain satisfactory. 
Thus with phosphates and borates the alkalies must be used very 
sparingly, or deterioration of the polished surfaces under the influence 
of the atmosphere is unavoidable. It was ascertained, however, that 
certain glasses, which in themselves were hygroscopic, could be made 
serviceable by the introduction of large percentages of clay, zinc oxide, 
or other compounds. Great care, nevertheless, had to be exercised in 
these cases, since, as a rule, very slight changes in the percentage 
composition were sufficient to induce a partial or complete crystallization. 
On this ground many of the apparently numerous possibilities of borate 
and phosphate glass had to be excluded. After surmounting various 
obstacles the authors succeeded in producing a series of phosphate- 
borate, and borosilicate glasses in small samples. For the phosphate, 
observation showed that magnesia, clay, and potash offered the least 
dispersion, so that a crown glass could be produced with dispersion far 
less than that seen in any yet made. The use of baryta with phosphoric 
acid in a crown glass caused a decrease of refractive index of 1-53 to 
1 • 59, and at the same time gave a lower dispersion. An aluminium- 
sodium-baryta-borate gave a borate crown glass, whose ray-path is 
useful for many purposes. The borate flint glasses, containing as much 
as 50 per cent, of boric acid, were made by the addition of clay, zinc 
oxide, and barium oxide, to satisfy all requirements, and to show no 
sign of hygroscopic characters. 
Apparatus for the production of material on a large scale had 
now to be prepared. A special form of stirrer had first to be devised. 
This was made of porcelain, and was provided with two cross-bars. 
Besides the ordinary motion of rotation, an up and down motion extending 
over 5-10 cm. was also simultaneously communicated to it by a special 
mechanism. In spite of this, apparently, very effective arrangement the 
homogeneity obtained was not sufficiently perfect to enable the greater 
part of the mass to be used in large pieces for telescopes. Corrosion 
of the porcelain of the vessel, evaporation from the upper surface, &c., 
caused groups of striae to form in the last portion poured out of the 
crucible. Accordingly it was determined to replace the porcelain 
crucible and stirrer by a crucible of platinum of 3 litre contents, and a 
platinum stirrer weighing about 1£ kgm. The result was, however, at 
first peculiarly unsatisfactory ; for, during the cooling, an extraordinary 
number of bubbles were developed, and after being used three or four 
times, the platinum of tbe crucible became brittle and cracked. Later 
experiments with a smaller and thicker platinum crucible, which showed 
no sign of cracks, and did not give rise to any gas-bubbles, proved the 
adaptability of platinum to the end in view. The use of platinum was 
only applicable to the borate glass, since phosphoric acid attacks the 
metal. 
The problem of the removal of the secondary spectrum having been 
