The largest refracting
telescope which the Clarks had yet constructed was one for the
University of Mississippi, which, on the outbreak of the civil war,
had come into the possession of the Astronomical Society of Chicago.
This would have been the last step, beyond which the firm would not
have been willing to go to any great extent, had it not happened that,
at this very time, a great telescope had been mounted in England.
This was made by Thomas Cooke & Sons of York, for Mr. R. S. Newall of
Gateshead on Tyne, England. The Clarks could not, of course, allow
themselves to be surpassed or even equaled by a foreign constructor;
yet they were averse to going much beyond the Cooke telescope in size.
Twenty-six inches aperture was the largest they would undertake.
I contended as strongly as I could for a larger telescope than
Mr. McCormick's, but they would agree to nothing of the sort,--the
supposed right of that gentleman to an instrument of equal size being
guarded as completely as if he had been a party to the negotiations.
So the contract was duly made for a telescope of twenty-six inches
clear aperture.

At that time Cooke and Clark were the only two men who had ever
succeeded in making refracting telescopes of the largest size. But in
order to exercise their skill, an art equally rare and difficult
had to be perfected, that of the glassmaker. Ordinary glass,
even ordinary optical glass, would not answer the purpose at all.
The two disks, one of crown glass and the other of flint, must be not
only of perfect transparency, but absolutely homogeneous through and
through, to avoid inequality of refraction, and thus cause all rays
passing through them to meet in the same focus. It was only about the
beginning of the century that flint disks of more than two or three
inches diameter could be made. Even after that, the art was supposed
to be a secret in the hands of a Swiss named Guinand, and his family.
Looking over the field, the Clarks concluded that the only firm that
could be relied on to furnish the glass was that of Chance & Co., of
Birmingham, England. So, as soon as the contracts were completed,
one of the Clark firm visited England and arranged with Chance &
Co. to supply the glass for the two telescopes. The firm failed in
a number of trials, but by repeated efforts finally reached success
at the end of a year. The glasses were received in December,
1871, and tested in the following month. A year and a