APPENDIX
431
automatic lamp where the arc has to be ‘ struck ’ or formed at
starting by touching the carbons together by a hand movement,
and then separating them, the lamp then remains burning, the
carbons as they consume away being pushed up by springs against
steel screws, somewhat after the manner of the ordinary carriage
candle lamps, and this lamp also performs well where the current
is steady and not too high in voltage ; but in practice I myself prefer
hand-fed lamps to any on account of their simplicity and freedom
from any liability to fail or become unsteady at a critical moment.
Of large automatic arc lamps little need be said ; those described
in Chapter XII. remain as the general pattern still in use. Messrs.
Oliver have brought one out on the same lines as the Brockie Pell
therein described, which is extremely steady and reliable, and is
in my opinion the best of such lamps at present on the market.
Messrs. Newton also make large hand-fed lamps for their
triple rotating and similar lanterns, and these are by some operators
preferred to even the ‘ Oliver ’ lamp.
Projection microscope.—Various slight improvements, both
optical and mechanical, have been made in this instrument since
1890. Of these the most important has been the introduction of a
quadruple condenser, with a view to still further reducing spherical
aberration in the lantern portion of the instrument. After many
years of experiment, the new condenser, as designed by my father,
reduces this fault to a minimum, and a considerable gain in both
light and definition is the result. With the new arrangement
apochromatic lenses especially show to more advantage than formerly.
Of lenses, several new ones have been brought out which perform
well with the projection microscope. Herr Reichert has intro¬
duced an oil-immersion lens, nominally \ but really 1 focus, of
N A 1*80 to 1’32, which for moderately high power is as good an
all-round projection-lens as I know of. As this lens lias a very great
working distance, it is as well to ask for a shortening of this if
required chiefly for projection, as the horizontal position tends to
drain the oil away.
The Zeiss apochromatic § of N a R40 does magnificent work,
but is very costly.
I can also speak from experience of Messrs. Swift’s Vj oil
immersion at 51. 5s. ; this also for all-round work can hardly be
beaten, and Messrs. Leitz also do a fine series at a moderate price.
Of mechanical improvements the most important has been the