June 19, 1890] 



NATURE 



187 



telescopes in many ways, but in some respects they are not so 

 good. They give images that are absohitely achromatic, while 

 the other form always has some uncorrected colour. They can 

 be made shorter, and as the light-grasping power is not reduced 

 liy the absorption of the glass of which the lenses are made, it 

 is in direct proportion to the surface or area of the mirror. 

 They have not had in many cases the same care bestowed upon 

 either their manufacture or upon their mounting as has been 

 L'iven in nearly every case to the refracting telescope. Speaking 



nerally, the mounting of the reflecting telescope has nearly 



ways been of a very imperfect kind — a matter of great con- 

 uquence, for upon the mounting of the astronomical telescope 

 so much depends. To direct the tube to any object is not dif- 

 ficult, but to keep it steadily moving so that the object remains 

 on the field of view requires that the tube should be carried by 

 an equatorial mounting of an efficient character. The first essen- 

 tial of such a mounting is an axis parallel to the axis of rotation 

 of the earth. The tube, being supported on this, will follow 

 any celestial object, such as a star, by simply turning the polar 

 axis in a contrary motion to that of the earth at the same rate. 

 If we make the telescope to swing in a plane parallel to the 

 polar axis, we can then direct the telescope to any part of the 

 sky, and we have the complete equatorial movement. There 

 are several ways in which this is practically done : we can have 

 a long open-work polar axis supported at top and bottom, 

 and swing the telescope in this, or we can have short strong 

 axes. As examples of the first, I will show you pictures of the 

 mountings designed for Cambridge and Greenwich Observa- 

 tories some forty years ago by Sir G. Airy, lately and for so 

 long our eminent Astronomer- Royal ; and as examples of the 

 other form, amongst others, the large telescope lately erected at 

 Nice, and also the larger one at Mount Hamilton, California, 

 now under the direction of Prof. Holden. 



The plan of bringing all the various handles and wheels that 

 control the movement of the telescope and the various acces- 

 sories down to the eye end, so as to be within reach of the 

 observer, is carried to the highest possible degree of perfection 

 here, as we can see by an inspection of the picture of the eye 

 end of this telescope. The observer with the reflecting telescope 

 is, with moderate-size instruments, never very far from the floor, 

 but in the case of the Lick telescope he might have to ascend 

 some thirty feet for objects low down in the sky ; but, thanks 

 to the ingenuity of Sir Howard Grubb, to whom the idea is 

 line, the floor of the whole Observatory is made to rise and fall 

 by hydraulic machinery at the will of the observer — a charming 

 but expensive way of solving the difficulty, as far as safety 

 goes, but not meeting the constant need of a change in position 

 as the telescope swings round in keeping up with the motion of 

 the object to which it is directed. The great length of the tube 

 of large refractors is well seen in this picture of the Lick tele- 

 scope ; it suggests flexure as the change is made in the direction 

 in which it points, and the consequent change of stress in the 

 different parts of the tube. 



The mounting of the reflector has been treated, if not so suc- 

 cessfully, with more variety than in the case of the refractor as 

 we shall see from the pictures I will show you, especially where 

 the Newtonian form is used. The 4-foot reflector at Melbourne 

 is mounted on the German plan, in a similar way to a refractor, 

 and an almost identical plan has been followed by the makers of 

 the 4-foot at the Paris Observatory. Lassell, who was the first 

 to mount a large reflector equatorially, used a mounting that may 

 be called the forked mounting, the polar axis being forked at its 

 upper end, and the tube of the telescope swinging between the 

 forks : a very excellent plan, dispensing with all counterpoising. 

 Wishing to obtain certain conditions that I thought and think 

 now favourable to the performance of the reflector, I devised a 

 mounting where the whole tube was supported at one end on a 

 bent arm ; a 3-foot mirror was mounted on this plan in 1879, 

 and worked admirably. The Newtonian form demands the 

 presence of the observer near the high end of the telescope, and 

 the trouble of getting him there and keeping him safely close to 

 the eye-piece is very great. As we see from the various photo- 

 graphs, several means have been employed to do this, none of 

 them quite satisfactory. 



All the refracting telescopes of note in the world are covered 

 l>y domes that effectually protect them from the weather ; these 

 domes are in some cases comparable in cost with the instruments 

 they cover. It is not surprising, therefore, that efforts have been 

 made to devise a means of getting rid of this costly dome and 

 the long movable tube. 



NO. 1077, ^'OL. 42] 



It was suggested many years ago that a combination of plane 

 mirrors could be used to direct light from any object into a fixed 

 telescope. This idea in a modified form has often been used 

 for special work, one plane mirror being used as we see in the 

 picture on the screen to throw a beam of light into a telescope 

 fixed horizontally ; for certain kinds of work this does admirably, 

 but the range is restricted, as can be easily seen, and the object 

 rotates in the field of view as the earth goes round. The next 

 step would be to place the telescope pointing parallel to the axis 

 of the earth and send the beam of light into it from the mirror, 

 which could now be carried by the tube so that by simply rotat- 

 ing the tube on its own axis the object would be kept in the field 

 of view. Sir Howard Grubb makes a small telescope on this 

 plan, and some years ago proposed a somewhat similar plan. 

 A sketch of this plan I will show you. You will see, however, 

 that here again the range is restricted, and, to use the telescope, 

 means would be required to constantly vary the inclination of 

 the small mirror at one-half the rate of inclination of the short 

 tube carrying the object-glass. 



By the use of two plane mirrors, however, the solution of the 

 problem of a fixed rotating telescope tube placed as a polar axis 

 is solved. By having such a telescope with a plane mirror at an 

 angle of 45" to the axis of the telescope in front of the object- 

 glass, we can, by simply rotating the telescope, see every object 

 lying on the equator ; and by adding another similar plane 

 mirror at an angle of 45° to the axis of the telescope, as bent 

 out at right angles by the first plane mirror^ and giving the 

 mirror a rotation perpendicular to this axis, we obtain the same 

 power of pointing the telescope as we have in the equatorial. 

 The idea of doing this was published many years ago, but it was 

 left to the skill and perseverance of M. Loewy, of the Paris 

 Observatory, to put it into practical use. Pie devised, and had 

 made, a telescope on this principle, of \o\ inches aperture, which 

 was completed in 1882. It has proved itself an unqualified suc- 

 cess, and many other larger ones are now being made in Paris, 

 including one of 23 inches aperture, now nearly completed, for 

 the Paris Observatory. 



A lantern copy of a drawing of this latter telescope will be 

 thrown on the screen, in order that you may see what manifest 

 advantages exist in this form of telescope. There is but one 

 objection that can be urged — that is, the possible damage to the 

 definition by the plane mirrors ; but this seems, from what I 

 have seen of the wonderful perfection of the plane mirrors made 

 by the Brothers Henry, to be an unreasonable one— at any rate 

 not an insurmountable one. In every other respect, except per- 

 haps a slight loss of light, this form of telescope is so mani- 

 festly superior to the ordinary form that it must supersede it in 

 time, not only for general work, but for such work as photography 

 and spectroscopy. 



ANNUAL VISITATION OF GREENWICH 

 OBSERVATORY. 



'T'HE Report of the Astronomer- Royal to the Board of 

 -*■ Visitors of the Royal Observatory, Greenwich, was 

 read at the Annual Visitation on June 7. The Report pre- 

 sented refers to the year 1889 May ii to 1890 May 10, 

 and exhibits the state of the Observatory on the last-named 

 day. 



With respect to astronomical observations it is noted that, 

 at the request of Dr. Gill, special attention has been paid to 

 the oppositions of the minor planets Victoria and Sappho. 

 Victoria has been observed 15 times on the meridian, and 

 Sappho 9 times ; while 244 observations have been made of 

 41 comparison stars for Victoria, and 151 observations of 42 

 stars for Sappho. At the request of Dr. Auwers, observations 

 of the Sappho stars will be renewed in the autumn of this year, 

 and an investigation made of the variation of personality with 

 magnitude, for use in reducing the observations to a uniform 

 system. 



The Lassell, south-east. Sheepshanks, and Shuckburgh equa- 

 torials are in good working order. Great difficulty has been 

 experienced at times in turning the south-east dome, and a 

 careful examination shows that this may be largely due to the 

 irregular shape of the cannon balls on which it rolls, and to a 

 sagging of the dome curb in some parts. 



The tube for the 28-inch refractor, which is of special con- 

 struction, has been made by Sir H. Grubb in preparation for the 

 object-glass which is now being figured. The experimental 



