August 25, 1923] 



NATURE 



287 



giant reflector of his- own construction made momentous 

 additions to our astronomical knowledge. 



Nearly sixty years later (1845) Lord Rosse ground, 

 polished, and mounted a six-foot reflector at Parsons- 

 town in Ireland. This leviathan of 54 feet focal length 

 was mounted somewhat after the fashion of Herschel's, 

 but solid masonry replaced the wooden-trestle struc- 

 ture. The movements of the tube were also similarly 

 restricted. 



While Lassel's reflectors, the largest of which was 

 four foot and made in 1863, were not an ad- 

 vance in size, yet he instituted a great im- 

 provement by mounting the instrument equa- 

 torially after the modified German type. 

 Grubb in 1870 completed a mirror of the 

 same dimensions for the Melbourne Obser- 



staging to accommodate the observer is therefore of 

 very complex construction, and the arrangements 

 adopted vary very considerably from one instrument 

 to another, no two forms being alike. 



We come now to the largest reflector of the present 

 time, namely the Hooker 100-inch erected at the Mount 

 Wilson Observatory in 1919. This mirror of 13 inches 

 thickness, and weighing four and one-half tons, has a 

 focal length of 42 feet. Though the block of glass was 

 cast in France, the figuring and silvering is due to the 



REFRACTORS 



vatory, mounting it in the composite fashion. 

 This was the last large reflector which em- 

 ployed a mirror of speculum metal, because 

 glass mirrors were beginning to supersede 

 them. 



In the years 1856 and 1857 Steinheil and 

 Foucault discovered a method of making 

 mirrors by depositing silver on glass surfaces. 

 This produced a highly efficient reflecting 

 surface and soon came into common use. One 

 of the first large reflectors with this type of 

 mirror was that made by Foucault himself for 

 the Paris Observatory. It was constructed 

 on the Newtonian principle, mounted equa- 

 torially on a heavy wooden framework 

 movable on castors and clock driven. In 

 1875 Martin made a four-foot mirror for the 

 same observatory, and it was only owing to 

 the thinness of the glass disc in relation to 

 its diameter that it was not a success. The 

 completed instrument was mounted in the 

 composite form. 



An immense advance was made by 

 Common, who in 1888 constructed and used 

 a mirror of five feet diameter. The tube was 

 mounted on the modified German plan, being 

 placed in a fork bolted to the upper end of 

 the polar axis. To minimise the great weight 

 of the polar axis on its bearings the novel 

 idea of floating it was adopted. It was not 

 till the year 1908, that is, twenty years later, 

 that a mirror of the same size was made. 

 This was accomplished by Ritchey for the 

 Mount Wilson Observatory : the style of 

 mounting was rather similar to that adopted 

 by Common. 



Another ten years saw the completion 

 (1918) of the six-foot reflector for the Dominion 

 Observatory, Ottawa. This great glass, the work of 

 Brashear, is equal in size to the speculum mirror of 

 Lord Rosse and weighs two tons. The form of 

 mounting of the tube is after the composite type, the 

 moving parts weighing 35 tons. The telescope is 

 capable of being used either as a Newtonian or as 

 a Cassegrainian. 



It should be noted that " rising floors " in an ob- 

 servatory cannot be employed for reflecting telescopes 

 of the Newtonian form because the eye end of the tele- 

 scope is situated at the upper end of the tube. The 



NO, 2808, VOL. 112] 



REFLECTORS 



iiiL:: -"ssssKHBKKKaiw;^ kkkksksh: K 



Fig. 3. — The growth of telescopes during the century 1820 to 1920. 



skill of Ritchey. The great tube carrying the mirror 

 is mounted after the English type, and the moving parts 

 of the telescope amount in all to fourteen and one-half 

 tons. Either the Newtonian or Cassegrainian form of 

 instrument can be utilised. Thus, after a lapse of two 

 and a half centuries, the one-inch reflecting telescope 

 of Sir Isaac Newton has grown into a monster of loo 

 inches. 



Having thus separately surveyed the progress of the 

 two types of telescopes, it is interesting to obtain a 

 bird's-eye view of this growth. This is represented by 



