30 Opaque Objects with High Powers. By G. W. Moorehouse. 
idea of the difficulties of interpretation accompanying ordinary 
methods of illumination may thus be obtained, both as produced 
by changes in focal adjustment and in position of mirror. 
Attempts have been made to use high powers on opaque 
objects by making pointed-nosed objectives, and also by construct- 
ing conical front lenses, and these methods have met with some 
degree of success with objectives as high as dry jths ; but as the 
finest views of the smaller objects and minute structural pecu- 
liarities can only he got with immersion objectives of wide angle of 
aperture, that necessarily have short working distances, other 
plans had to be invented. One proposition was to throw the light 
down one tube of a binocular instrument and view the object in 
the other tube, thus making the instrument its own illuminator. 
Previously, Professor Smith had devised a plan by which the 
objective itself was made the illuminator, light being reflected 
downward into it by means of a small silver speculum. The light 
was admitted to the speculum through an aperture in the tube, 
or an adapter above the objective. As all are aware, several 
opticians have made more or less important modifications of this 
device. The one I have used is described in Carpenter, fifth edi- 
tion, page 153, under the name of Beck’s Vertical Illuminator. 
It is simple, cheap, easily removed and cleaned, and admirably 
suited to the purpose for which it was made. It consists of a 
disk of thin glass attached to a milled head, and placed in an 
adapter fitted with the Society screw, and placed between the 
objective and the body of the instrument. The light entering the 
aperture is reflected by the glass disk downward into the objective, 
and by it concentrated upon the object. 
In using the instrument, as a general thing, the lamp should 
be placed about eight inches from the aperture, and opposite it, 
but, on some objects, it is desirable to change both distance and 
position. The German student’s lamp is poorly adapted for this 
work, and I have found the ordinary flat wick or sun-burner to 
serve a much better purpose. The lamp should be placed with 
the flame edgeways toward the aperture, and the narrowest pos- 
sible image of the flame brought to the centre of the field of view. 
The image of the flame as seen in the instrument should be so 
perfect that any flickering of the blaze may he distinctly seen. 
A small bright image of the flame may be seen on the under sur- 
face of the slide with a pocket magnifier. The mirror under the 
stage may be used with advantage to find and centre the object, 
and at times illumination, both from above and below, may be used 
advantageously in determining the character of the study. 
Dr. Carpenter speaks of the vertical illuminator being “ spe- 
cially applicable to diatoms, Polycystina, minute foraminifera, and 
the scales of insects, viewed under objectives of from T 4 <yths to ith 
