Mat 1, 1894.] 



KNOWLEDGE 



99 



Speaking generally, it may be taken that the whole weight 

 of the tube, mirror and cell will probably be some forty or 

 fifty tons, and in the present state of engineering skill it 

 does not at first sight appear to be so very difficult to 

 mount this on a simple universal stand, which will enable 

 it to be pointed to all parts of the heavens. But such a 

 mounting would be insufficient unless the telescope is to 

 be used simply for getting transitory glimpses of celestial 

 objects. It would be practically useless for all serious 

 work, bearing in mind the requirements of the future ; for 

 the chief hope of progress in astronomical research lies 

 undoubtedly in the application of photography. It would 

 appear to be not only useless (except to satisfy mere 

 popular curiosity), but an absolute waste of money and 

 energy to build an instrument of such lai-ge dimensions, 

 unless its light-grasping power can be utilized for celestial 

 photography. 



The mounting must be of the equatorial form, and the 

 problem to be solved is how to carry the enormous weights 

 involved with all the accuracy and delicacy which are so 

 essential for stellar-photographic work, while at the same 



Fig. 



1 . — Section of the Telescope Tube, showing the doublet sides, and the direction 

 of the air currents which will be produced by the fan or blower. 



time the instrument must also be readily available for 

 general visual work. To satisfy the latter condition, it is 

 necessary above aU things, as has been pointed out by 

 Dr. Common, that the eye-piece of the instrument should 

 not be at any great distance from the centre of motion. 

 This, in the ordinary Newtonian form, necessitates serious 

 modifications, for there would have to be a considerable 

 distance between the centre of motion and the centre of 

 gravity of the essential parts. The telescope would thus 

 be considerably out of balance, and dead-weight would have 

 to be added to correct this. The instrument, no matter 

 what its length and weight may be, must be carried as 

 evenly and acctirately as any of the smaller instruments 

 specially constructed for stellar photography, and this may 

 be expressed by saying that the pointing of the telescope 



must never be in error by an amount greater than the 

 motion of a star in one-twentieth of a second of time. 



It was, no doubt, in view of the very great difficulties 

 of fultilhng these conditions that Dr. Common was led to 

 believe that it would be necessary to revert to the alt- 

 azimuth form in mounting such large reflectors. 



Now, when we consider that the lowest magnifying 

 power of an eight-foot reflector is about four hundred and 

 eighty, and of a ten -foot (such as is proposed for the 

 next Paris Exhibition) six hundred, it will be understood 

 how very unsatisfactory such a mounting would be for 

 large-size telescopes. 



To view objects in a telescope satisfactorily, it is 

 necessary to bring them into or near to the centre of the 

 field of view ; but even suppose we are satisfied to view 

 them while in any part of the field of an ordinary eye-piece, 

 the object could only be viewed in the eight-foot telescope 

 for about fifteen seconds, and in the ten -foot telescope for 

 twelve seconds at a time. This would render the instru- 

 ment practically useless. 



The larger the telescope the more important it is to 

 have it equatorially mounted, and driven 

 correctly by clockwork, so that the observer 

 may watch for a favourable opportunity for 

 distinct vision. It is to be remembered that, 

 in the case of very large apertures, it is 

 only in glimpses on a fine night that good 

 definition is to be obtained ; and therefore 

 it is all the more important that the ob- 

 server should have the opportunity of 

 watching for and taking advantage of these 

 favourable moments. 



I have ventured, in a paper read before 

 the Royal Dublin Society on February 21st, 

 to shadow forth what I believe will be the 

 most hopeful principle on which to mount 

 a monster reflecting telescope. 



Dr. Common himself has made a splendid 

 advance in adopting the system of flotation 

 of the polar axis ; this principle of flotation 

 appears to me to be capable of further de- 

 velopment, and I have given some thought 

 to the matter. It is perfectly possible to 

 make a tube for a Newtonian reflecting tele- 

 scope (which is necessarily closed at the 

 lower end) of such a weight, and with its 

 weight so distributed, that it will not only 

 float submerged in water to a certain point 

 (preferably near the upper end), but will be 

 in a state of equilibrium when placed at any 

 or in every position down to a certain angle, 

 the angle depending on the exact outside 

 form of the tube. For instance. Fig. 1 

 shows the tube closed at its lower end and 

 perfectly symmetrical round its axis. The total weight of 

 the tube must be equal to the weight of water which is dis- 

 placed when the tube is sunk to the centre of the sphere; 

 the weight of the different sections along the axis can be so 

 distributed that the tube will equally well remain in any 

 position, except it be so far turned over that the cylindrical 

 part of the tube is lifted out of the water at one end and 

 dipped at the other. 



By making the spherical part of about the proportions of 

 the figure, the tube can be depressed to within twenty-five 

 degrees of the horizon, and still remain in perfect equi- 

 librium. 



Now suppose the tube to have a pair of trunnions 

 attached at the water-line, and these to be carried on a, 

 polar axis of, say, the English type (see Fig. 2). We then 



