6] 2 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



of tin soldered on a rim of iron, about 14 inches broad, and -i. thick, the dia- 

 meter of which is equal to the iron ring which holds the speculum. On the 

 flat part of the rim is cemented, all around, some close-grained cloth of an equal 

 breadth with the rim. The cover has 1 handles near the upper end, and under 

 them 2 flaps that project about an inch and are 6 inches broad. When the 

 cover is hung or laid on the speculum, so that the 2 flaps are close to the ring 

 which incloses it, the rim of the cover, as far as it is lined with cloth, will rest 

 against the edge of the iron ring, aiid fit it all around very closely. 



To take oflr" and put on again the cover, a small ladder is provided, which 

 being set at the outside against the back of the tube, the person who is to un- 

 cover it goes up, and descends into the tube by means of a board with steps. 

 This board goes across the mirror in a parallel direction with it, and being nar- 

 row, does not interfere with the work of loosening the screws to take them ofi-'. 

 When they are removed, the person comes out of the tube the same way, still 

 leaving the speculum covered, but when at the top of the ladder brings out the 

 inside board-steps. The 2 handles of the cover now present themselves at the 

 back, so that 2 persons can easily lift it off, without suffering it to touch the 

 mirror in any place. It must theai immediately be carried into the observatory, 

 and remain there till the mirror is to be covered again ; but first of all the inner 

 and outer cover of the tube ought to be carefully closed up. When the specu- 

 lum is to be covered again, great care is required to see that no drops of dew 

 may fall from the outer cover of the tube on the inner one ; or at least that 

 these may not find their way to the mirror ; and to let the first object be to put 

 its own cover on it before any thing be done about fixing it there. 



A slider, on an adjustable foundation, is planted at the mouth of the telescope, 

 so as to be directed towards the centre of the mirror. It carries a brass tube, 

 into which all the single eye-glasses, or micrometers, are made to slide. When 

 they are nearly brought to the focus, a milled head under the end of the tube 

 turns a bar, the motion of which adjusts them completely. The focus of the 

 great mirror is directed to its proper place, by putting 2 plates with springs on 

 the rim that limits the aperture of the tube, into 2 places which are marked. 

 Then a cap with a small hole being put into the sliding tube, an assistant with 

 a proper handle must screw in or out one or other of the adjusting screws at the 

 back of the mirror, till the plates on the aperture in front of the telescope be- 

 come both visible ; for they are contrived so that when the mirror is not pro- 

 perly adjusted, either one or both will vanish. At the same time these plates, by 

 their situation, serve to inform us which of the screws, whether that to the right 

 or that to the left, is in fault, by which means the adjustment becomes a very 

 easy operation. 



