. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . of Lord Crawforri (then Lord Liudsay) at Dun Echt<Aberdeenshire) about 1873. The plan is now almost universal ylloDt-vd. Telescopes of such dimensions can be conveniently l:li directed to any object by the circles without the observer beingunder the necessity to climb a special ladder. But when muchlarger instruments are required the hour circle becomes inaccessiblefrom tho floor, and means have to be devised for read-ing both circles from the eye end. This was firstaccomplished by Grubb in the great
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . of Lord Crawforri (then Lord Liudsay) at Dun Echt<Aberdeenshire) about 1873. The plan is now almost universal ylloDt-vd. Telescopes of such dimensions can be conveniently l:li directed to any object by the circles without the observer beingunder the necessity to climb a special ladder. But when muchlarger instruments are required the hour circle becomes inaccessiblefrom tho floor, and means have to be devised for read-ing both circles from the eye end. This was firstaccomplished by Grubb in the great refractorof 27-inche3 aperture which he constructedfor the Vienna observatory, represented insection in fig. 22. The observers eye isapplied to the small telescope E, which(by means of prisms numbered 1, 2,3, 4) views the vernier attached tothe cross-head simultaneouslywith the hour circle attachedto the upper end of tho polaraxis. Light to illuminatetho vernier and circle isthrown from the lampL upon prism 4 bythe prisms 6 and5. Prism 1 is inthe axis of thedeclinationcircle andalways. Fio. 22.—Grubb3 27-inch refractor (Vienna). reflects rays along that axis, whatever the position of the telescop*may be, whilst the prisms .2, 3, 4, 5, and 6 are attached to the cross-head and therefore preserve their relative positions to each the eye-piece of the bent telescope E another hour circleattached to the lower end of the polar axis can be seen ; thus bnassistant is able to direct the telescope by a handle at H to a»ydesired hour angle. A slight rotatory motion of the telescopn Eon its axis euables the vernier of the declination circle to be i cadthrough prism 1. The leading features of this fine instruraentrepresent those of all Grubbs largo telescopes. The mode <S re-lieving the friction of the axis is similar to that em-ployed in the llelbourue tele-scope and in the accouut of (haVienna telescope publishedby Grubb. The end fric-tion of the polar ax
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