246 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
central light the diaphragm is shifted little by little out of the centre, 
and after each movement its aperture is again illuminated by slightly 
displacing the mirror, or, better, the lamp. 
In English and American instruments the oblique illumination is 
easily regulated by rotating the condenser L about the object itself O as 
centre. A hemispherical lens L' placed beneath the object and optically 
united to the slide by an immersion-liquid, collects, without sensible 
deviation, the convergent rays from the condenser L, whatever may be the 
inclination of the latter to the axis x y of the Microscope (fig. 24). As a 
rule the aperture of the illuminating pencil ought not to exceed the third 
of the aperture of the objective, but cases may occur in which the employ- 
ment of pencils of large amplitude may be of service, as e. g. when very 
small coloured particles, dispersed in a feebly transparent but colourless 
medium, have to be distinguished ; for in this case, although when using 
a very wide pencil the images disappear, yet the opposition of the colour 
will remain. 
Fio. 24. 
By employing an illuminating pencil so oblique that it does not 
enter into the aperture of the objective, the field will be dark, but any 
particles or objects in the field can so modify the direction of the rays 
that they penetrate into the objective, and a bright image will be ob- 
tained on a dark ground. This method of illumination gives very beau- 
tiful effects, but it is only employed with objectives of low or moderate 
power. 
The Analysing Eye-piece.*— Mr. W. Lighton remarks:— “At the 
first meeting of the American Society of Microscopists, held at Indiana- 
polis, I presented a paper upon an analysing eye-piece, and exhibited 
one that I had been using for several years. I sent, at a later date, a 
drawing and description of it to the San Francisco Microscopical Society. 
* Amer. Mon Ivlicr. Journ., xiii. (1892) pp. 260-2. 
