Forty-feet Refecting Telescope. 407 
is lined with cloth, will rest against the edge of the iron ring, 
and fit it all around very closely. 
In six places are painted white marks which divide the cir- 
cumference equally ; and six claw-screws are provided, of the 
shape that is represented in fig. 47. These are applied to the 
six marked places. The end a being put over the iron ring 
b to take hold of the back, the screw c is then fastened so as 
to press upon the outside of the cover and rim, till the lining 
of it is brought into close contact with the iron ring. 
To take off 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 uncover it goes up, and de- 
scends into the tube by means of a board with steps. This 
board goes across the mirror in a parallel direction with it, 
and being narrow, does not interfere with the work of 
loosening the screws to take them off. When they are re- 
moved, 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 two handles of 
the cover now present themselves at the back, so that two 
persons can easily lift it off without suffering it to touch the 
mirror in any place. It must then 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 speculum is to be covered again, great care is re- 
quired to see that no drops of dew may fall from the outer 
cover of the tube upon the inner one ; or at least that these 
may not find their way to the mirror; and to let the first 
3 Ga 
