BIRMINGHAM NATURAL HISTORY SOCIETY. 
139 
them with lenses of ordinary make but of very fine quality, and 
the result was very much in favour of the new form. Compen¬ 
sating eye-pieces were used with all the objectives tested, as they 
give better results, even with objectives of the ordinary construction, 
than the Huyghenian form generally used. The apochromatics were 
tested on a variety of objects with the deepest eye-pieces, and there 
was not the slightest indication of their breaking down. To get a 
better idea of the superiority of these lenses, the objects were first 
of all viewed with some objectives of the old construction, but of 
very fine quality. These were tried with the highest eye-pieces 
they would bear, and were very satisfactory, one very fine quarter- 
inch running the apochromatic of about the same power pretty close ; 
but there was no mistaking the superiority of the latter under the 
deepest eye-pieces, all the minutest details coming out perfectly 
sharp and clear to the last. Of course, the illumination was of the 
best, a prism being used instead of the plain mirror, and the first 
class condenser was most carefully adjusted before commencing 
the trial, the performance of an objective being very much improved 
in proportion as these details are attended to. The objectives 
referred to were dry lenses, and in power equivalent to an English 
inch and quarter-inch. Another advantage of these lenses is the 
large working distance, and the facility with which the magni¬ 
fying power can be increased without disturbing the object under 
examination. 
Owing to their more perfect corrections, these lenses are 
specially adapted for photo-micrography, which is becoming more 
and more indispensable as a means of scientific research. The 
chemically prepared film of the photographic plate is capable of 
recording faint markings and minute structural details, which 
cannot be detected in the microscopic image by the human eye, 
or at least very imperfectly. As an illustration of this, from an 
astronomical point of view, the following facts with respect to the 
discovery of small planetary bodies which move in orbits between 
those of Mars and Jupiter may be interesting:—“ From observa¬ 
tions of the first of these, Ceres, in 1801 until the end of 1891, 321 
had been discovered by the comparatively laborious method of eye 
June, 1893 . 
