398 SUMMARY OF CURRENT RESEARCHES RELATING TO 
Fig. 43. 
to replace the inconvenient blow-pipe arrangement of the old in- 
strument. 
Fig. 43 represents the burner used when only slight changes oi 
temperature are needed. It gives a very small blue flame, and is itself 
so small as only slightly to interfere 
with the brightness of the field of 
view. In the other form of burner 
(fig. 44), used for higher tempera- 
tures, the gas issues from a ring- 
shaped slit. It is closed beneath 
by a thin plate of glass or mica, so 
that the flame is driven towards the centre by the draught thus pro- 
duced. The brightness of the field of view is not sensibly affected by 
the passage of the rays 
Fig. 44. through this transparent 
plate and the thin layer 
of burning gas within the 
ring. 
As a further improve- 
ment of the Microscope, the 
divided circle is enclosed 
in the object-stage, so that 
it is protected from dirt 
and injury from acid va- 
pours, &c. The scale is 
read directly from above by 
means of a window in the 
upper side of the stage. 
For experiments at 
very high temperatures, 
the larger instrument described in the Zeitschr. f. Instrumentenk., 1884, 
p. 369, is necessary. Into this several improvements have been intro- 
duced. In order to effect a greater concentration of heat upon the 
object, and to diminish the heating of the metal parts of the Microscope, 
the blow-pipe flame is directed through a chimney of asbestos, bound 
with brass, which can be fitted into the opening of the stage. A new 
form is given to the water screen protecting the objective. It consists 
of a socket 2 cm. long, with double walls and strong copper base, which 
fits tightly over the objective, and expands at its upper edge into a 
disc of about 5 cm. diameter. To prevent the condensation of water 
upon the objective, an arrangement is added by which a stream of air is 
directed upon it. 
Improvements have also been made in the Projection Microscope 
described in this Journal, 1887, p. 291. The indiarubber tubes of the 
old instrument are replaced by metal ones. For cooling the alum solu- 
tion, a spiral tube conveying a stream of cold water is used instead of 
the water screen. The mirror is not rigidly fixed as before, but can 
be turned about a hinge and fixed by a binding screw several degrees 
from its normal position of 45°. By this means a uniform brightness 
can be maintained when, by changes in the electric arc, the illumina- 
tion of the field of view has slightly shifted. For cooling the 
