1890.] on Astronomical Telescopes. 169 



lately and for so long our eminent Astronomer-Eoyal ; 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 accessories 

 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. Thanks to the ingenuity of Sir Howard Grubb, to 

 whom the idea is due, the whole of the floor of the Observatory is made 

 to rise and fall by hydraulic machinery at the wdll 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 re- 

 fractors is well seen in this picture of the Lick telescope : 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 success- 

 fully, 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 con- 

 ditions 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 tele- 

 scope, 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 

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

 them quite satisfactory. 



All the refracting telescopes of note in the world are covered by 

 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. 



It was suggested many years ago that a combination of plane 



