778 SUMMARY OF CURRENT RESEARCHES RELATING TO 
satisfactorily settled by the production of the phosphate and borate glass, 
attention was now directed to the improvement of the ordinary silicate 
glass. By the introduction of boric acid, zinc oxide, magnesia, baryta, 
and clay, silicate crown and light flint glasses were obtained with very 
varied relations of refraction and dispersion. Many combinations of 
light crown glass and light baryta flint glass were found to be of great 
service in aplanatic constructions for photographic purposes : for, owing 
to their increased transparency for the chemically active rays and their 
slight dispersion, they gave plane and sharp images in the camera. 
The effect of strain produced in glass on cooling had next to be con- 
sidered. Ten years ago Dr. Schott had made some experiments in this 
direction in relation to the so-called “ Hartglas.” He proved that the 
hardening of the glass proceeds inwards layer by layer so that, the 
internal part solidifying later than the outer, a state of strain was 
induced which manifested itself by diminished specific gravity and 
phenomena of double refraction. 
Now the lowering of the density of a substance is constantly accom- 
panied by a diminution in the refractive index, so that this subject 
naturally becomes of the utmost importance in relation to delicate 
optical instruments. A strain becomes especially harmful when it is 
developed not in the centre of an objective, but excentrically. The 
effect of this is that the focal length varies in different parts just as if it 
were not spherical, but irregularly cut in the direction of the strain. 
Repeated experiments, made in the kind of furnace hitherto used, showed 
that the ordinary technical process of cooling no longer corresponded 
to the requirements of the improved kinds of glass. Accordingly, two 
years ago other experiments were undertaken. Success was attained by 
automatically regulating the source of heat to which the glass was sub- 
jected, and by very gradually allowing it to fall in temperature. Hitherto 
a thick cylindrical copper cauldron had been used as a cooling vessel. 
This was placed in the course of a large gas flame, and was connected 
with a mercury vapour pressure thermometer. In the improved form of 
apparatus, the expansion of the mercury was used not only to give the 
temperature, but also to regulate the flame. By this means it was possible 
to keep up a constant temperature over long periods of time, and also 
to effect a very gradual fall in temperature down to that at which 
experience has shown that the movement of the smallest particles 
ceases. The maximum temperature at which any of the glasses lost 
all signs of strain, and entered into a very slightly soft condition, 
w r as 465° ; and on the other hand, the minimum temperature at which 
any of them hardened was about 370°. This fall of 95° was then 
extended over periods varying from a few days up to four weeks, and 
more successful results of cooling were obtained than by the old method 
under the most favourable conditions. 
On the Removal of the Chromatic Difference of the Spherical 
Aberration in Microscope Systems.* — Dr. Arthur Kerber states that 
for systems of three lenses, the chromatic difference of the magnification 
is removed by having the upper lens under-compensated, and the middle 
one strongly over-compensated. By dispersion of the white ray a b 
Central-Ztg. f. Optik u. Mech., xi. (1890) pp. 217-9. 
