474 



NA TURE 



[September 13, 1900 



and the Greenwich telescope of 28 inches, are mounted on the 

 German form. Some of these carry a reflector as well, as, for 

 instance, the telescope lately presented to the Greenwich Obser- 

 vatory by Sir Henry Thompson, which, in addition to a 26-inch 

 refractor, carries a 30-inch reflector at the other end of the 

 declination axis, such as had been previously used by Sir 

 William Huggins and Dr. Roberts ; the last, and perhaps the 

 finest, example of the German form being the Yerkes telescope 

 at Chicago. 



The small reflector made by Sir Isaac Newton, probably the 

 first ever made, and now at the Royal Society, is mounted on a 

 ball, gripped by two curved pieces, attached to the body of the 

 telescope, which allows the telescope to be pointed in any direc- 

 tion. We have not much information as to the mounting of 

 early reflectors. Sir William Herschel mounted his 4-foot tele- 

 scope on a rough but admirably-planned open-work mounting, 

 capable of being turned round, and with means to tilt the tele- 

 scope to any required angle. This form was not very suitable 

 for picking up objects or determining their position, except 

 indirectly ; but for the way it was used by Sir William Herschel 

 it was most admirably adapted : the telescope being elevated to 

 the required angle, it was left in that position, and became 

 practically a transit instrument. All the objects passing through 

 the field of view (which was of considerable extent, as the eye- 

 piece could be moved in declination) were observed, and their 

 places in time and declination noted, so that the positions of all 

 these objects in the zone observed were obtained with a con- 

 siderable degree of accuracy. It was on this plan that Sir John 

 Herschel made his general catalogue of nebulae, embracing all 

 the nebulae 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 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, pulleys 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 Rosse mounted his tele- 

 scopes 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 Cassegrain, the observer is practically on the ground level. 

 A somewhat similar instrument 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 can- 

 not 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 (Lockyer, "Star-gazing," p. 453). 



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 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, 



NO. 161 I, VOL. 62] 



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 helio- 

 stat 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 equatorial mountingj^ 

 and the rising floor are not wanted. The mechanical arrange- 

 ments of importance are confined to the mounting of the neces- 

 sary machinery to carry the large plane mirror and move it 

 round at the proper rate. The telescope need not have any 

 tube (that to the Paris telescope is, of course, only placed there 

 for effect), as the flimsiest covering is enough if it excludes false 

 light falling on the eye-end ; and, more important than all, the 

 observer sits at his ease in the dark chamber. This question of 

 the observer, and the conditions under which he observes, is a 

 most important one as regards both the quality and quantity of 

 the work done. 



We have watched the astronomer, first observing from the 

 floor level, then mounted on a high scaffold like Sir William 

 Herschel, Lassell and Lord Rosse; then, starting again fron> 

 the floor level and using the early achromatic telescope ; then, 

 as these grew in size, climbing up on observing chairs to suit the 

 various positions of the eye-end of the telescope, as we see ir> 

 Mr. Newall's great telescope ; then brought to the floor again 

 by that excellent device of Sir Howard Grubb, the rising floor. 

 This is in use with the Lick and the Yerkes telescopes, where 

 the observer is practically always on the floor level, though 

 constant attention is needed, and the circular motion has to be 

 provided for by constant movement, to say nothing of the danger 

 of the floor going wrong. Then we have the ideal condition, as 

 in the Equatorial Coude at the Paris Observatory, where the 

 observer sits comfortably sheltered and looks down the tele- 

 scope, and from this position can survey the whole of the visible 

 heavens. The comfort of the observer is a most important 

 matter, especially for the long exposures that are given to photo- 

 graphic plates, as well as for continued visual work. In such a 

 form of telescope as that at Paris the heliostat form of mounting 

 the plane mirror is most suitable, notwithstanding the rotation 

 of the image. But there is another way in which a plane mirror 

 can be mounted, and that is on the plan first proposed by 

 Auguste many years ago, and lately brought forward again by 

 Mons. Lippman, of Paris, and that is by simply mounting the 

 plane mirror on a polar axis and parallel therewith, and causing 

 this mirror to rotate at half the speed of the earth's rotation. 

 Any part of the heavens seen by any person reflected from this 

 mirror will appear to be fixed in space, and not partake of the 

 apparent movement of the earth, so long as the mirror is kept 

 moving at this rate. A telescope, therefore, directed to such a 

 mirror can observe any heavenly body as if it were in an 

 absolutely fixed position so long as the angle of the mirror shall 

 not be such as to make the reflected beam less than will fill the 

 object-glass. There is one disadvantage in the coelostat, as this 

 instrument is called, and that is its suitability only for regions 

 near the equator. The range above and below, however, is large 

 enough to include the greater portion of the heavens, and that 

 portion in which the solar system is included. Here the tele- 

 scope must be moved in azimuth for different portions of the sky, 

 as is fully explained by Prof. Turner in vol. Ivi. of the Monthly 

 Notices, and it therefore becomes necessary to provide for moving 

 the telescope in azimuth from time to time as different zones 

 above or below the equator are observed. No instrument yet 

 devised is suitable for all kinds of work, but this form, notwith- 

 standing its defects, has so many and such important advantages 

 that I think it will obviate the necessity of building any larger 

 refractors on the usual models. The cost of producing a tele- 

 scope much larger than the Yerkes on that model, in comparison 

 with what could be done on the plan I now advocate, renders it 

 most improbable that further money will be spent in that way. 

 It may be asked. What are the lines of research which could be 

 taken up by a telescope of this construction, and on what lines 

 should the telescope be built ? I will endeavour to answer this. 

 All the work that is usually done by an astronomical telescope. 



