August 2 5, 1923] 



NA TURE 



2% 





focal length ; for experience had shown that the greater 

 the focal length the less the colour. 



Thus J about the year 1680 we come to the age of 

 giant telescopes, when their lengths measured anything 

 from 60 feet to 210 feet. These cumbrous instruments 

 were generally suspended by their middle from tall 

 masts or towers, and to reduce their weight diaphragms 

 placed at stated intervals took the place of wooden tubes. 

 Thus were the telescopes of Hevelius. Huyghens 

 adopted the novel principle of only placing the object 

 glass on the mast, the eyepiece being attached to 

 it by a long cord which could be stretched tight, and 

 so make the proper optical alignment. 



An illustration of a giant observa- 

 tory of Hevelius's time given here 

 (Fig. i) displays three of these long 

 telescopes in use. Mechanism is shown 

 by which not only can the telescopes 

 be hoisted into position but the roof of 

 the tower to which the telescopes are 

 suspended can also be turned round 

 to neutralise the earth's motion. The 

 illustration shows that even in those 

 days a considerable observatory staff 

 was necessary. 



A telescope cannot be properly 

 manipulated unless it is equatorially 

 mounted, i.e. mounted on an axis 

 inclined to the latitude in which it is 

 used. One of the first, if not the first, 

 telescope to be set up in this manner 

 was that used by Scheiner in 1618 for 

 observing the spots on the sun. 

 Scheiner had only to direct the tele- 

 scope to the sun, and fix it in decHna- 

 tion, when the diurnal movement 

 could be compensated by simply mov- 

 ing the telescope westward by hand. 

 The form of mounting he adopted 

 was the foundation of the German 

 type of mounting telescopes, to which 

 reference will be made later. 



Not only is it imperative for a 

 telescope to be equatorially mounted, 

 but it must also be driven by some 

 power, clockwork or otherwise, so 

 that the object under observation 

 will always remain in the centre of 

 the field of view of the telescope. Hooka, so far as is 

 known, was the first to adopt this principle in 1674. 

 As is indicated in an old print of his instrument, he 

 mounted his quadrant at the upper end of a long polar 

 axis, and rotated this by means of gear wheels actuated 

 by a falling weight. The speed was controlled by a 

 conical pendulum governor, which could be shortened 

 or lengthened at will. We had to wait, however, until 

 the year 1823 before a really efficient driving clock 

 was applied to a telescope. This was the work of 

 Fraunhofer, and was adapted to the gj-in. Dorpat re- 

 fractor, the largest refractor of that period, made for the 

 Czar Nicholas of Russia. The principle is the same as 

 that used to-day ; the clockwork, driven by weights 

 and controlled by a governor, actuating a tangent screw 

 which is in gear with the threads cut in the circum- 

 ference of the driving circle to which the telescope can 



NO. 2808, VOL. I 12] 



be clamped. The Dorpat instrument may be said to 

 be the first real modern refractor, as it embodied all 

 the fundamental features of telescopes constructed 

 afterwards. 



There are three well-known recognised forms of 

 mounting telescopes, illustrated in Fig. 2, and termed 

 the " EngHsh," " German," and " Composite " types. 

 In the English type the telescope tube is mounted 

 directly on the polar axis midway between the supports 

 of this axis, and being symmetrically placed balances 

 itself both in Right Ascension and Declination. The 

 composite type is rather similar to that of the English, 



COMPOSITE 



Fig. 2. — The three main methotis of mounting telescopes. 



only the tube is placed on one side of the polar axis 

 and the counterpoise weights on the opposite side. In 

 the German type, the tube with its counterpoise weights 

 is fixed symmetrically to the prolongation of the upper 

 end of the polar axis, that is, outside the supports of 

 this axis. There is still a more modern modification 

 of the German type, in which the polar axis is prolonged 

 at its upper end, taking the shape of a fork. The 

 telescope tube is placed symmetrically in this fork, 

 thus obviating the necessity for counterpoise weights. 



Coming now to the advance in telescope construction, 

 mention only may be made of such instruments as the 

 15-inch Pulkowa (1839) by Merz and Mahler, the 15-inch 

 Harvard (1847), ^^so by Merz and Mahler, and the 

 18-inch Chicago University telescope (1862), by Alvan 

 Clark. 



The year 1868 saw the completion of the fine 25-inch 



