I therefore
set to work again on the other disc of glass, to try if I could
finish it in such a way as to excel the first one. After nearly
a year's work I found that I could only succeed in equalling it.
But then, during this time, I had removed the working of mirrors
from mere chance to a fair amount of certainty. By bringing my
mathematical knowledge to bear on the subject, I had devised a
method of testing and measuring my work which, I am happy to say,
has been fairly successful, and has enabled me to produce the
spherical, elliptic, parabolic, or hyperbolic curve in my
mirrors, with almost unvarying success. The study of the
practical working of specula and lenses has also absorbed a good
deal of my spare time during the last two years, and the work
involved has been scarcely less difficult. Altogether, I
consider this last year (1882-3) to mark the busiest period of my
life.
"It will be observed that I have only given an account of those
branches of study in which I have put to practical test the
deductions from theoretical reasoning. I am at present engaged
on the theory of the achromatic object-glass, with regard to
spherical chromatism--a subject upon which, I believe, nearly all
our text-books are silent, but one nevertheless of vital
importance to the optician. I can only proceed very slowly with
it, on account of having to grind and figure lenses for every
step of the theory, to keep myself in the right track; as mere
theorizing is apt to lead one very much astray, unless it be
checked by constant experiment.
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