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II. — A New Mode of Illuminating for High Powers. 
By Dr. Whittell. 
I HAVE lately been testing some high-power objectives, and have 
discovered by accident a mode of illuminating which I have never 
seen described, and which appears to be worth further trial. 
I was working in a room lighted by a window to my left, which 
is protected by a balcony so as to admit the sun's rays during the 
winter months only. The microscope was Beck's larger model, 
with rectangular prism and achromatic condenser, the central rays 
being stopped. The object-glass was Powell and Lealand's xVth, 
with immersion front. The test-object was the Surirella gemma, 
and I had just succeeded in getting a fair view of the so-called 
longitudinal markings, when the sun peeped from behind a cloud 
and shone brightly on the microscope. The object became iridescent 
and the outline rather indistinct, but I could still distinguish the 
markings. I turned the prism towards another part of the sky, 
but found that this did not affect the appearance. I could scarcely 
imagine that the sun's rays from above could affect the view of an 
object seen under a glass working so close to the cover as a rVth, 
but by way of experiment I shut off all light from below. I was 
surprised to see the object, although not very brightly illumined, 
showing all the markings as before. I now took the small con- 
denser for opaque objects which fits into the stand, and condensed 
the sun's rays upon the upper surface of the cover. The field 
became bright, and the object and its markings beautifully distinct. 
The object was not white upon a dark field, as in what is called 
dark-ground illumination, but had the same appearance as when 
lighted by oblique light from below. The markings, however, were 
more distinct. I substituted the dry front for the immersion one, 
and found the result equally good. 
I have since tried the same experiment with one of Zeiss' ^Vth 
immersion lenses, with splendid results. With this lens I found 
that after obtaining a fine view of the longitudinal markings, I 
could, by slightly altering the focus, cause them to disappear, and a 
series of dots running transversely took their place. 
I have also tried the experiment with one of Beck's |-inch 
objectives, and with No. 2 eye-piece I had a good view of the 
longitudinal lines. 
If any microscopist be desirous of testing the observation, I 
must caution him not to be satisfied with a mere dark-ground 
illumination, but to gently toy with the condenser until he obtain 
a field having all the appearance of being illuminated from below. 
In my last experiment I found the condensing lens close to the 
