132 THE MICROSCOPICAL NEWS. 
examining one of Powell and Lealand’s high-class ,jths 
(homog.). The aperture varied from “open” to “closed” 1°36 
to 1°42. The working distance was ‘oo7inch., while the Leitz 
vs, which would not work over a// our slides, had a working 
distance of only ‘oos5. Our Powell and Lealand water immersion 
1 of 1°20 numerical aperture has a working distance of ‘oo4, 
and can only, therefore, be used with the thinnest covers. The 
advantage of oil immersion in increasing the working distance will 
therefore be seen. With the Leitz’s ,1, homogeneous immersion 
we have lately spent much time. The podura scale illuminated 
with the immersion paraboloid shows out splendidly, while the 
Amphipleura pellucida, used with the vertical illuminator, is 
resolved without any difficulty. The collars of monads are well 
shown, and specially good are the shows of Bacillus tuberculosis and 
B. anthracis. 
Amphipleura pellucida is a good exercise for the student, and he 
should not be satisfied until he is able to bring out the markings 
clearly. The Leitz’s ,4, homogeneous is capable of doing this, 
but we venture to say that not one student in ten will be able to 
display “the hateful markings” until some one has shown him the 
“knack.” 
Illumination has more to do with bringing out the structure of 
objects than some microscopists imagine, and here is good exercise 
for them. 
EVENINGS WITH THE MICROSCOPE. 
I. 
E will commence by supposing every reader a beginner in 
microscopic studies, and to be the possessor of a microscope, 
an eye-piece micrometer, a common paraffin lamp, and a bull’s- 
eye condenser. Perhaps he is also possessed of many accessories, 
but for the present anyhow we will dispense with these things, and 
only add to our stock as we progress with studies of difficulty. 
A beginner nearly always is exceedingly anxious to be the 
possessor of some slides; well, this is only natural, and so we will 
indicate what his first two slides should be. It is very seldom that 
such slides are purchased in the first instance by the student, but 
if our advice be followed there is no doubt but that he will find 
progress in the microscopic art to be easy and rapid. These first 
slides which can be purchased for a few shillings should be :— 
1. A stage micrometer, ruled preferably on the French and 
English systems, in one slide; and, 
