1901] MICROSCOPICAL JOURNAL. 165 
will then give an aplanatic cone exceeding ‘90. Such is 
the case with Watson & Son’s holoscopic condenser. 
Again, the top lens can be removed and a condenser of 
low power secured. Oil immersion condensers are too 
little appreciated, and it will be found, if it is desired to 
work with medium and high powers, that the oil immer- 
sion system will serve every purpose and is practically a 
universal condenser. 
ILLUMINATION WiTH ARTIFICIAL Ligut.—The lamp that 
has proved most universally satisfactory is the regular 
one sold for microscopical work, with a 3-inch or $-inch 
wick, but to many people this is objectionable for several 
reasons, the chief of which is that with the general use 
of gas and electric light, a mineral oil is not kept in the 
house, excepting for this special lamp; it also is not clean 
to handle, and requires a certain amount of attention ; also 
it is not always immediately ready for service when re- 
quired. In laboratories, such a lamp is out of the ques- 
tion, and bare gas jets, or gas jets with upright chimneys, 
are generally to be found. 
I have recently been making some experiments with gas 
and electric lamps to see if some practical form of illum- 
inant, always available for use without special prepara- 
tion, cannot be devised for critical microscopical work. 
Two important considerations have to be kept in view, 
one is that the light must be brilliant, and the other is 
that it should be possible to focus an image of the source 
of Jight by means of the substage condenser, in the field 
of view. 
A very serviceable illumination can be secured with the 
Welsbach incandescent gas light, but the reticulations 
of the mantle are an obvious objection, and the flame has 
too large a surface. These can be overcome by means of 
a shade of metal surrounding the chimney at a distance 
of three or four inches. In this shade,a small rectangu- 
lar or circular slot should be perforated. When working, 
