RICHARDSON, ON STOPS FOR OBLIQUE ILLUMINATION. ll 
After numerous trials I was successful in forming some 
stops for my Smith and Beck’s achromatic condenser, with 
the assistance of three of which, in particular, the markings 
of many diatoms requiring 
oblique illumination can be 
quickly and beautifully exhi- & © 
bited, and with a field, I 
may say altogether, free from 
the glare and milkiness so 
often experienced with the 
mirror, as well as with the 
prism. 
Ido not know a more exqui- 
site microscopic object than a 
properly mounted P. Hippocampus, seen with stop third of 
the illustration and Smith and Beck’s 1th objective. In fact, 
nothing could be more perfect than the definition of both 
sets of lines on that diatom with the above combination. 
Again, P. angulatum, which requires a nicer management of 
the light than P. Hippocampus, with the mirror, condenser, 
and ordinary disc stop, is instantly resolved clear and sharp 
with the same objective, and either stops second, third, or 
sixth of the illustration. The fourth stop will be found 
useful for exhibiting diatoms with only one series of lines, 
longitudinal or transverse. And the first and fifth are also 
adapted to illumination of some diatoms with the so-called 
double lines. The first likewise gives a very excellent black 
ground with low powers, and is a useful stop for examining 
Coscinodiscus, Arachnoidiscus, Heliopelta, Triceratium, &c. 
As the only high powers of English makers I have used were 
the 1th and 1th of Smith and Beck, I cannot, of course, say 
how the stops would act with their higher objectives, or with 
the glasses of other English opticians ; but I can confidently 
recommend the second, third, and sixth particularly to those 
microscopists who possess Smith and Beck’s powers above 
mentioned, for to my eye, at least, the definition of their 1th 
and 1th with those stops could scarcely be exceeded. 
It is essential for the most perfect illumination with the 
stops, they should be so arranged that they can be rotated. 
The tube, therefore, which carries them ought only to slide 
in that of the achromatic combination, and should also be of 
sufficient length to project inferiorly, in order that it may be 
easily got at for rotation and for rapid change of stops, if 
required. 
The projecting extremity should be provided with a milled 
rim, at least half an inch wide. 
