‘200 
MR. LISTER ON ACHROMATIC COMPOUND MICROSCOPES. 
wholly detached from theory, of facts that have been in part very recently 
ascertained. In investigating them I have depended chiefly on magnified 
measurements and diagrams, which, though not strictly correct, may perhaps 
be as well adapted as other far more exact but difficult methods, for construct- 
ing the minute and complex object-glass of a microscope. It is to this that 
the observations made particularly apply ; and should they bring more within 
reach than previously, the requisites which have been enumerated, I trust 
they may not be unacceptable to the enlightened optician ; especially if in this 
department he unite, like Tulley, the zeal of the amateur with the skill of the 
superior artist. 
I intend soon to put to the test of experiment, whether or not the principle 
of the two aplanatic pencils may be applied to telescopes, in cases in which it 
is requisite to restrict their length, so as to enlarge their aperture with a corre- 
sponding increase of light and distinctness. In the mean time, it would give 
me pleasure to see that principle demonstrated, if it deserve it, by some abler 
hand than mine, and treated in a more rigorous manner than my own limited 
acquaintance with mathematical science qualifies me to undertake. 
