Stargazing: Past and Present

by Sir Norman Lockyer

Fig. 139.—Mr. Grubb’s form applied to a Cassegrain Reflector.

The clock is seen to the north of the pillar. While this is driving the telescope, rods coming down to the eyepiece enable the observer to make any small alterations in right ascension or declination; indeed in all modern instruments everything except winding the clock is done at the eyepiece, so that the observer when fairly at work is not disturbed. The lamp to illuminate the micrometer wires is shown near the finder. The friction rollers, which take nearly all the weight off the surfaces of the polar axis, are connected with the compound levers shown above the casing of the polar axis.

In Fig. [139] we have Mr. Grubb’s revision of the German form. The pillar is composite, and the support of the upper part of the polar axis is not so direct as in the mounting which has just been referred to. There are, however, several interesting modifications to which attention may be drawn. The lamp is placed at the end of the hollow polar axis, and supplies light not only for the micrometer wires, but for reading the circles; the central cavity of the lower support is utilised for the clock, which works on part of a circle, instead of a complete one, as in the instrument already described.

In the case of Newtonian reflectors the observer requires to do his work at the upper end of the tube; this therefore should be as near the ground as possible. This is accomplished by reducing the support to a minimum. Figs. 140 and 141 show two forms of this mounting, designed by Mr. Grubb and Mr. Browning.

The two largest and most perfectly mounted refractors on the German form at present in existence are those at Gateshead and Washington, U.S. The former belongs to Mr. Newall, a gentleman who, connected with those who were among the first to recognise the genius of our great English optician, Cooke, did not hesitate to risk thousands of pounds in one great experiment, the success of which will have a most important bearing upon the astronomy of the future.

Fig. 140.—Grubb’s form for Newtonians.

Fig. 141.—Browning’s mounting for Newtonians.

In the year 1860 the largest refractors which had been turned out of the Optical Institute at Munich under the control, first, of the great Fraunhofer, and afterwards of Merz, were those of 177 square inches area at Poulkowa and Cambridge (U.S.). Our own Cooke, who was rapidly bringing back some of the old prestige of Dollond and Tulley’s time to England—a prestige which was lost to us by the unwise meddling of our excise laws and the duty on glass,[^[16]] which prevented experiments in glass-making—had completed a 9⅓ inch for Mr. Fletcher and a 10 inch for Mr. Barclay; while in America Alvan Clarke had gone from strength to strength till he had completed a refractor of 18½ inches for Chicago. The areas of these objectives are 67, 78·5, and 268 inches respectively.