SCIENCE-GOS SIP. 215 
MICROSCOPY FOR BEGINNERS. 
By F. Suimtincron Scares, F.R.M.S, 
( Continued from paye 184.) 
The achromatic condenser of 1 N.A. is greatly 
superior to the Abbé illuminator mentioned previously. 
The aperture is nominally smaller, but its aplanatic 
cone frequently exceeds .7 N.A., and it has the 
additional advantage of being achromatic. We 
unhesitatingly advise its selection if the student can 
afford the extra cost. Prices vary with different makers. 
We may add that this condenser gives also excellent 
results in photography. The immersion condensers 
now sold by the leading makers, two of which have 
been noticed recently in Scrence-Gossiv, have 
important uses and advan- 
tages that are only now 
receiving due recognition 
from workers generally. 
They are. however, expen- 
sive, and not really neces- 
sary to the beginner, for 
whose guidance these 
papers are primarily 
written. We may add that 
the top lens of all con- 
densers is generally remov- 
able so as to give a larger 
illuminated field when a 
low power lens is being used. The mount of the con- 
denser will either be carried by the focussing and 
centering stages mentioned when dealing with stands, 
or will slide in a short tube-ring affixed to the under 
side of thestage. Of course the former is much prefer- 
able, not only in the facilities it gives for focussing, 
but also for centering; though at least one maker pro- 
vides a non-focussing sub-stage ring, that is fitted 
ACHROMATIC CONDENSER. 
CARRIER AND Spots. 
with centering screws, and can also be readily 
swung aside when not wanted. Here again, how- 
ever, cost will probably enter into the matter. 
Very often there is a ring beneath the mount 
into which various stops can be placed to give oblique 
illumination (necessary for certain diatoms and similar 
work), or to give dark ground illumination by means 
of a central black spot, varying in size with the 
objective used, and which acts by cutting out the central 
rays of light, leaving the marginal rays to be refracted 
into the objective by the object on the slide. This 
gives most beautiful effects, but is not often used for 
critical work. The spot-lens proper, and the 
paraboloid, are not now much used. 
An iris diaphragm is very gr 
convenience to any form of diaphragm plate, whether 
it be a revolving plate or a series of stops. Under 
any circumstances the diaphragm should be tencath 
and not above the condenser, as we have seen it in 
certain microscopes. Polarizing apparatus, though 
giving beautiful effects, is only useful for certain 
classes of work, mainly petrological or crystallogical. 
It can be added at any time if required. The 
bull’s-eye condenser on the ordinary stand is too 
well known to need description here. Its cost is 
from 6s. to 15s. As ordinarily made it is nearly 
hemispherical, and its chromatic and spherical 
aberrations are most marked. It can be used with 
transmitted light to strengthen the illumination from 
a lamp, in which case its plane side should be turned 
rather near to the flame, the adjustment being made 
so as to allow almost parallel light to fall upon the 
mirror. A piece of white paper is a useful assistant in 
doing this. For opaque objects requiring illumination 
on the stage (reflected light), where either artificial light 
or daylight is the source ot illumination, it is usual to 
place the flat side of the bull’s-eye near the object and the 
spherical side at somedistance from the lamp, ifa lamp 
“atly superior in 
Sipe Sitver Rervecror. 
be used. Some little practice is necessary in these 
matters at first, as well as in the management of the 
condenser proper, to which we shall allude more par- 
ticularly later on. For objects to be viewed by 
reflected light the side silver reflector is very con- 
venient, but costs about 25s. 
If the tailrod carrying the concave mirror will 
allow of the latter being adjusted above the stage, 
excellent illumination can be got by this means, as 
mentioned in SciENCE-GossiP, Vol. V., page 377- 
