318 Sir Howard Gruhb [May 25, 



3. It may be urged that the friction of the water will prevent the 

 rapid setting' of the instrument. In a telescope of this size all the 

 motions would he effected by motors of some description, guided by 

 the observer from a commutator-board at the eye end, and there would 

 be no difficulty in setting the telescope quite as quickly as could 

 be expected considering its great size. 



4. It may be objected that currents will be set up in the water by 

 the moving of the telescope, which currents will affect the steadiness. 

 No doubt this will be the case to some extent, but these will soon 

 subside, and the motion necessary for following the stars will be so 

 slow that no perceptible effect of this kind will be felt from it. 



As to convenience in getting at the eye end, there need be no 

 difficulty whatever in this form. As the eye-piece is only jibout 15 

 fett from the centre of motion, the movement of the observer is never 

 more than 3 feet per hour. By means of a platform such as that 

 shown in Fig. 2, running on rails, and quite independent of the 

 instrument, the eye end is readily accessible at all times. To over- 

 come the rotation of the tube as the instrument moves in right 

 ascension, I would pierce the tube for eye-pieces every 30° round its 

 circumference, and mount the flat mirror and cell in a collar so as to 

 enable it to be readily rotated through intervals of 30°. By these 

 means the image of the celestial object to be observed could be sent 

 through either or any of the perforations of the tube, and the observer 

 always observe in the direction most convenient to himself. 



There are various difficulties about this construction which may 

 naturally suggest themselves, but there are none, I believe, which 

 cannot be overcome. 



This is hardly the place to discuss details, but if there bo anj 

 here who are sufficiently interested in this new form of equatorial to 

 desire further information, I would refer them to an article in the 

 present month's issue of ' Knowledge ' which deals with most of the 

 difficulties. 



Putting aside now the question of reflectors versus refractors, 

 there are some directions, applicable equally to reflectors and re- 

 fractors, in which it is evidently possible to improve our designs for 

 large equatorial instruments. It is not the first time that I have 

 uro-ed similar developments, but the advantages of what I recom- 

 mended were not so apparent then as they are now in the present 

 advanced state of astronomical research. 



It may be remembered that when I lectured here in the year 

 1886 I strongly urged the desirability of employing some form of 

 motive power for carrying out the various motions required in 

 manipulating the large equatorials, domes, &c, and exhibited a model 

 design I had made for the Lick Observatory, which illustrated the 

 manner in which these various motors could be controlled by the 

 observer, as well as the then newly devised lifting floor arrangement. 

 The latter, that is the lifting floor, was, as you are aware, adopted by 

 the trustees of the Lick Observatory, and I suppose we may assume 

 that it was considered a success, as it has been copied in the case of 



