OCTOBEE 19, 1900.] 



SCIENCE. 



601 



focus telescopes we have just spoken of. 

 The object-glass was at a height, the re- 

 flector was close to the ground. There was 

 a tube to one 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, puUej's and ropes surrounded 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, and this 

 he mounted in this way. Lord Kosse 

 mounted his telescopes somewhat after the 

 manner of Sir William Herschel. The 

 present earl has mounted a 3-foot equa- 

 torially. 



A 4-foot telescope was made by Thomas 

 Grubb for Melbourne, and this he mounted 

 on the German plan. The telescope being 

 a Cassegrain, the observer is practically on 

 the ground level. A somewhat similar in- 

 strument exists at the Paris Observatory. 

 Lassell's 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 combination 

 of a reflector and a refractor. I 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 sug- 

 gested by Hooke, who, at a meeting of the 

 Royal Society, when the difficulty of mount- 

 ing the long-focus lenses of Huyghens was 

 under discussion, pointed out that all diffi- 

 culties 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, 



*Lockyer, Star-gazing, p. 453. 



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 im- 

 portant advantages. Chief amongst these 

 that the observer sits in perfect comfort 

 at the upper end of the polar axis, whence 

 he need not move, and by suitable arrange- 

 ments 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 photographs 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. 

 But in 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 equa- 

 torial mounting and the rising floor are not 

 wanted. The mechanical arrangements of 



