Extract from the “English Mechanic,” September 3rd, 1880.
“The telescope is a silvered-glass Newtonian reflector. There are two specula, one of 18 in. in diameter, figured by the celebrated With, in 1878, and is a magnificent specimen of that eminent maker’s handiwork. In fact, I am given to understand that Mr. Cooper Key considered that he had attained perfection in this mirror, and of all men then living there was no better judge than he. The other is 18-1/4 in. in diameter, and was originally figured by Mr. Cooper Key himself; but it was only perfect over the interior 14 in., and, consequently, while the whole aperture could be used on the moon and planets, this was all of it that could be used upon close doubles. I determined, therefore, to give this mirror to Mr. Calver to re-figure, and, if possible, excel Mr. With’s speculum.
“In order that Mr. Calver might know the extent of his task, I returned home through London and Chelmsford, bringing with me both mirrors, which Mr. Calver tested for himself at the centre of curvature. After the trial was over, he pronounced Mr. With’s mirror to be an exceptionally fine one, and heartily congratulated me on the possession of such a beauty. I left the other one with him to be re-figured.
“And now to come to the most important part of all—the performance of the 18 in. With mirror upon close double stars. Since the telescope came into my possession, I have never had a thoroughly good night; but even on an indifferent one, I have, with the full aperture, divided such extremely difficult tests as η Coronæ, μ2 Herculis, and δ Cygni; and these results were quite sufficient to show me that the very high reputation of its excellent maker is worthily and proudly maintained by this mirror. In fact, the definition it gave one night, when the Scottish Astronomer Royal was staying with me of ε Boötis, was such, that the Professor told me I could not expect any telescope, reflector, or refractor, upon the same object, and on the same night, to do better. However, I knew Mr. Calver was doing his very utmost in the honourable and praiseworthy competition to attain perfection, to give me a mirror which would bear favourable comparison with Mr. With’s. It reached me about a week ago, and on the same evening I had an opportunity of testing it upon what I consider to be one of the very severest tests that can be applied to any telescope, reflector or refractor, in the northern heavens, that is δ Cygni[3]. To make the test as severe as possible, I applied the whole aperture at once, with a power of 347. I was surprised and delighted beyond measure with the result. The small star stood out clearly separated from its bright primary, and there were two or three diffraction rings round the latter unbroken in any part, and equally bright all round. I consider the results which Mr. Calver has attained in this mirror cannot possibly be excelled. I measured this star on two separate nights, and the mean results are pos. 328·9°, dist. 1·29″. I also measured, with this mirror, another difficult star of the same character, namely, with a bright primary and small companion, ζ Herculis, with the following results, pos. 111·3°, dist. 1·03″. I make the companion of this star a dusky blue, or lavender. As regards the light-grasping power of the mirror, I turned it, the other night, on the minute pair between β1 and β2 Capricorni, which Sir John Herschel gave as the test of the power of any telescope and eye to observe the satellites of Uranus. The night was very unsteady, but the minute stars of the pair were most conspicuous.”
Footnote:
[3] Note.—Mr. Perry does not mean that δ Cygni is one of the most difficult tests as regards the distance of the components. He has often seen it with his 6-1/2″ Silvered Glass Reflector, and even with a stop of 5-1/2″ on the same instrument; but he was surprised at the beauty and perfection of the images—perfect and unbroken rings—and he knew that these appearances could not have presented themselves had not the figure of the mirror been of extraordinary perfection.
From Thos. Westlake, Esq.
Fordingbridge,
June, 1880.