MICROSCOPIC TEST OBJECTS. 
139 
most difficult, tests. It is impossible, however, with such an 
arrangement to bring out the finest markings with ease 
and rapidity. We might as well attempt to feed every 
stomach with one mouth as to feed every power with one- 
aperture. A -J-inch, J-th, and -^th require apertures of very 
different lengths, and hence some easy mechanical contrivance 
is necessary for effecting a micrometrical adjustment of the 
length of the aperture, and thus securing the necessary com- 
mand over the size of the illuminating pencil. The well-known 
flute-key adjustment in my own “ kettledrum,” or double- 
hemispherical condenser, answers this purpose perfectly. The 
two hemispherical lenses, used also by Webster, Davis, Highley, 
and other observers, are, as it were, the mere raw material; the 
secret of success lies in the proper management of the con- 
densed and convergent pencils. In the diaphragm cap of the 
single hemisphere the apertures were always of one and the 
same size, and hence arose a practical difficulty in using it for 
delicate tests. Some little coaxing of the light by means of a 
bull’s-eye condenser between the lamp and the kettledrum was, 
however, an easy method of modifying the intensity of any 
particular pencil ; but the final flute-key adjustment of the- 
little sliding diaphragms gives, with the utmost nicety, just 
that length of aperture which the test under examination re- 
quires. The practical value of this fine adjustment is so obvious 
that few condensers with similarly pierced diaphragms are 
without some similar apparatus for regulating the intensity of 
the illuminating pencil. 
THE ACHROMATIC CONDENSER. 
It will be admitted by all that the condensers of the present 
day are a very great improvement on the first and early pro- 
duction of Dr. Wollaston. While, as to powers, the Wollaston 
doublet and Holland’s triplet were grand improvements on 
Dollond’s single lenses — the best powers of my youthful days — 
we had long to wait before the subject of illumination was 
practically or theoretically investigated. Here opticians and 
microscopists were equally at fault, and when Dr. Wollaston 
recommended for an illuminating lens one of three-fourths of an 
inch in focal length, in which the microscopic object was placed 
in a vortex of foci, where the rays crossed in a thousand points 
both before and after they fell upon the object, he failed to 
realise the true method of illumination. The presence of both 
chromatic and spherical aberration in such a construction would 
be fatal to success. The condenser of Wollaston was neverthe- 
less received as an improvement over the ordinary methods of 
illumination, and its chief fault was subsequently remedied by 
Dujardin, who contrived an instrument which he termed art 
