188 
BIRMINGHAM NATURAL HISTORY SOCIETY. 
correct estimate of the object under examination, it is essential to 
know something of the laws of optics which are concerned in the 
production of a true microscopic image. The utilisation to the full 
extent of the powers of the microscope can, therefore, only be 
acquired by long and patient study of those principles on which its 
correct performance depends, and also on the skilful and effective 
manner in which the objects examined by it are prepared and 
mounted. It has been the aim of the Sub-Section to impart this 
knowledge ; and several members who are experts have been good 
enough to give practical demonstrations on the Friday evenings, 
but thev have not hitherto met with much encouragement. It 
cannot be expected that these gentlemen will continue to give their 
time, and go to considerable trouble in preparing and bringing 
down material and apparatus, if those whom they are desirous of 
helping do not show by their attendance that they appreciate the 
efforts that are made. 
While on microscopical matters, I should like to refer to 
the new apochromatic objectives of Zeiss. These lenses have 
been in use some little time, but, as far as I am aware, have 
not been exhibited or described at any of our meetings. A 
special kind of glass, which possesses a wider range in refraction 
and dispersion than the old crown and flint glass, is used in the 
construction of these objectives, together with the introduction of 
special fluorite lenses into the combination. By this means it is 
possible to entirely remove the secondary spectrum, leaving only a 
residuum of a tertiary character, which is got rid of by specially 
constructed eye-pieces, called compensating eye-pieces, the result 
being that images perfectly free from colour are obtained. One 
of the many advantages which these lenses possess is that the 
power can be increased by specially constructed eye-pieces to a 
much greater extent than is possible with the old lenses. By this 
means one objective can be made to do the duty of several ; and 
the microscopist of the future will only require about three objectives 
to do everything he wants. 
Through the courtesy of Dr. Anthony, I had an opportunity of 
testing the performance of two of these objectives, and comparing 
June, 1893. 
