Transactions of section a. 669 



the positions of all these objects in the zone observed were obtained with a 

 considerable degree of accuracy. It was on this plan that Sir John Herschel made 

 his general catalogue of nebulae, embracing all the nebulaj he could see in both 

 hemispheres ; a complete work by one man that is almost unique in the history of 

 astronomy. 



Sir William Herschel's mounting of his 4-foot reflector differs in almost every 

 particular from the mountings of the long-focus telescopes we have just spoken of. 

 The object-glass was at a height, the reflector was close to the ground. There was 

 a tube to o.ne telescope, but not to the other. The observer in one case stood on 

 the ground, in the other he was on a stage at a considerable elevation. One pole 

 sufficed with a cord for one ; a whole mass of poles, wheels, pulleys, and ropes sur- 

 rounded the other. In one respect only were they alike — they both did fine work. 



Lassell seems to have been the first to mount a reflector equatorially. He, 

 like Herschel, made a 4-foot telescope, which he mounted in this way. Lord 

 Rosse mounted his telescopes somewhat after the manner of Sir William Herschel. 

 The present Earl has mounted a 3-foot equatorially. 



A 4-foot telescope was made by Thomas Grubb for Melbourne, and this he 

 mounted on the German plan. The telescope being a Cassegrajn, the observer is 

 practically on the ground level. A somewhat similar instrument exists at the 

 Paris Observatory. Lassell'.^ 4-foot was mounted in what is called a fork mounting, 

 as is also my own 5-foot reflector, and this in some ways seems well adapted for 

 reflectors of the Newtonian kind. 



We now come to the Paris telescope. This is really the result of the combina- 

 tion of a reflector and a refractor. 1 cannot say when a plane mirror was first 

 used to direct the light into a telescope for astronomical purposes. It seems first 

 to have been suggested by Hooke, who, at a meeting of the Royal Society, 

 when the difficulty of mounting the long-focus lenses of Huyghens was under 

 discussion, pointed out that all difficulties would be done away with if, instead of 

 giving movement to the huge telescope itself, a plane mirror were made to move 

 in front of it.^ 



The Earl of Crawford, then Lord Lindsay, used a heliostat to direct the rays 

 from the sun, on the occasion of the transit of Venus, through a lens of 40 feet 

 focal length, in order to obtain photographs, and it was also largely used by the 

 American observers on the same occasion. 



Monsieur Loewy at Paris proposed in 1871 a most ingenious telescope made by 

 a combination of two plane mirrors and an achromatic object-glass, which he calls 

 a Coude telescope, which has some most important advantages. Chief amongst 

 these are that the observer sits in perfect comfort at the upper end of the polar 

 axis, whence he need not move, and by suitable arrangements he can direct the 

 telescope to any part of the visible heavens. Several have been made in France, 

 including a large one of 24 inches aperture, erected at the Paris Observatory, and 

 which has already made its mark by the production of perhaps the best photog^raphs 

 of the moon yet obtained. I have already spoken of Lord Lindsay and his 40-foot 

 telescope, fed, as it were, with light from a heliostat. This is exactly the 

 plan that has been followed in the design of the large telescope in the Paris 

 Exhibition. Rutin place of a lens of 4 inches aperture and a heliostat a few inches 

 larger, the Paris telescope has a plane mirror of 6 feet and a lens exceeding 

 4 feet in diameter, with a focal length of 186 feet. The cost of a mounting on the 

 German plan and of a dome to shelter such an instrument would have been 

 enormous. The form chosen is at once the best and cheapest. One of the 

 great disadvantages is that from the nature of things it cannot take in the 

 whole of the heavens. The heliostat form of mounting of the plane mirror causes 

 a rotation of the image in the field of view which in many lines of research is a 

 strong objection. There is much to be said on the other side. The dome is 

 dispensed with ; the tube, the equatoi'ial mounting, and the rising floor are not 

 wanted. The m-'chanical arrangements of importance are contined to the 

 mounting of the necessary machinery to carry the large plane mirror and move 

 it round at the proper rate. The telescope need not have any tube (that to 



' Lockyer, Star-gaziing, p. 453, 



