286 



NA TURE 



[August 25, 1923 



made by Cooke for Mr. R. S, Newall's observatory at 

 Gateshead. This instrument, by far the largest of its 

 day, was mounted after the German form. It had a 

 focal length of 30 feet, so that the dome and observing 

 chair had to be of great proportions. So satisfactory 

 was the behaviour of this instrument, that, after a 

 journey to this country to inspect this telescope, the 

 representatives from the Washington Observatory 

 ordered a 26-inch of 32-feet focal length from Alvan 

 Clark, and it was completed in 1873. 



In 1880, Grubb surpassed this size by making a 

 27-inch for the Vienna Observatory ; but five years 

 later (1885) Alvan Clark turned out a 30-inch objective 

 of 42-feet focal length for the Pulkowa Observatory. 

 The following year (1886) saw another objective of the 

 same size constructed by the Brothers Henry for the 

 Nice Observatory, but this was soon eclipsed by the 

 completion in 1887, by Alvan Clark, of the Great Lick 

 Refractor of 36-inch aperture and 57-feet focal length, 

 erected on Mount Hamilton, in California. For this 

 instrument an observing chair, as such, had to be 

 abandoned, but the floor of the observatory was made 

 capable of elevation and depression, thus avoiding 

 many difficulties and adding great facilities. 



Two large telescopes, though not records in size, 

 followed the construction of the Lick instrument. The 

 first was the 28-inch of 28 feet focal length by Grubb 

 (1893) for the Greenwich Observatory, mounted after 

 the English fashion ; this is the largest refractor in 

 Great Britain to-day. In the following year (1894) 

 the Brothers Henry completed the 32-inch, 53 feet 

 focal length telescope for the Astrophysical Observatory 

 at Meudon, near Paris. 



The largest refractor in the world to-day, namely the 

 Yerkes telescope of the University of Chicago, was 

 completed in 1895, the object glass by Alvan Clark 

 being 40-inch in diameter and of 62-feet focal length. 

 It is mounted very similarly to the Lick instrument and 

 fitted with all the latest facilities for assisting the 

 observer at the eye end, including a rising and falling 

 floor. 



In mounting large refractors the standard forms 

 have occasionally been departed from in order to attain 

 some special end. Four examples of these may be 

 mentioned here. Thus at the Paris Observatory there 

 was erected in 1890 a 23J-inch objective of 62-feet 

 focal length in a tube mounted in the " Coude " form 

 after the design of Monsieur Loewy. This instrument 

 is so constructed that the observer is housed in a 

 comfortable room in which the eye end of the telescope 

 is suitably fixed, and he can observe any object in the 

 heavens, without moving from his chair, by means of 

 reflections from two mirrors in the peculiar shaped tube 

 after the light has passed through the object glass. 



Another novel form was exhibited at the Paris 

 Exhibition of 1900 to utilise an object glass of 49 inches 

 aperture and 197 feet focal length made by Monsieur 

 Gautier. In order to avoid the necessity of having to 

 move such a heavy object-glass and tube, the principle 

 adopted was to place the telescope horizontally in a 

 true north and south position with the object-glass 

 facing north. The light from any celestial object was 

 then reflected into this tube by means of a large silver- 

 on-glass mirror mounted as a siderostat and moved by 

 clockwork. This particular telescope has never been 



NO. 2808, VOL. I 12] 



effectively used, so in spite of its great objective it has 

 not been classed as the greatest telescope of to-day. 



A curious mounting is that employed for the 27-inch 

 telescope of 70 feet focal length of the Treptow 

 Observatory, near Berlin, erected in 1909. The main 

 object in the construction was to obviate the cost of 

 a large dome and rising floor, and also to make the eye- 

 piece of the instrument very easy of access for numerous 

 visitors. To accomplish this, the tube was erected on 

 the modified German type of mounting in such a way 

 that the eye end of the tube should be situated just 

 above the upper end of the polar axis. The tube was 

 counterpoised in declination by two great weights 

 placed at the extremities of two long arms extending 

 northwards and symmetrically placed as regards the 

 tube. Thus the eyepiece was in the centre of motion 

 of the telescope and practically stationary for all 

 positions of the tube : also by simply setting the tube 

 near the position of horizontality it could be covered 

 by a light wooden low structure. 



The last novel form of mounting to be mentioned was 

 erected in 1912, and is known as the 150-foot Tower 

 Telescope of the Mount Wilson Observatory. Its 

 origin developed from the fact that an objective of 

 long focal length was required to be used in conjunction 

 with a spectroscope also of long focal length. 



Previous experience had shown that air currents 

 near the ground affected the definition when such long 

 instruments were used in a horizontal position. Dr. 

 G. E. Hale conceived the idea of mounting the object 

 lens high up on a metal girder tower and throwing the 

 images of the celestial object to be studied vertically 

 downwards on to the spectroscope placed vertically in 

 a shaft in the ground, employing two plane mirrors 

 above the object-glass to reflect the object downwards. 

 The actual lens in use has an aperture of 12 inches and 

 a focal length of 150 feet, while the focal length of the 

 spectroscope is 75 feet. One of the chief peculiarities 

 of the construction was that the girder work of the 

 Tower was really in duplicate, one within the other 

 and not touching at any place. While the dome at the 

 top rested on the outer casing, the mirrors and lens 

 were supported by the inner one ; thus any wind 

 pressure which might set up vibration in the outer 

 casing did not affect the inner casing which supported 

 the optical parts of the arrangement. 



Reference has previously been made to the various 

 forms of reflecting telescopes, such as the Newtonian, 

 Gregorian, Cassegrainian, and Herschelian, and to the 

 first reflector ever made, namely, that by Sir Isaac 

 Newton in 1668. For a long time the progress of this 

 type of telescope was slow, but impetus was given to it 

 by Sir William Herschel, who was the first to make 

 mirrors of really large dimensions. The mirrors them- 

 selves were composed of speculum metal — an alloy of 

 copper and tin and highly polished. Herschel's largest 

 reflector was four feet in diameter, with a focal length 

 of forty feet. It was erected at Slough, near Windsor, 

 in the year 1789. The tube was pivoted near the 

 ground and mounted between high wooden trestles : 

 while there was no restriction to its movement in the 

 vertical direction it was only capable of a ver>- small 

 lateral motion east and west of the meridian. Just as 

 Galileo, w^ith his pigmy refractor, revolutionised ideas 

 with his wonderful discoveries, so Herschel with the 



