35° Dr. Herschel j s Description of a 
upon which by degrees the whole structure was raised as it 
now stands ; and the speculum being highly polished and put 
into the tube, I had the first view through it on Feb. 19, 1787. 
I do not however date the completing of the instrument till 
much later; for the first speculum, by a mismanagement of 
the person who cast it, came out thinner on the centre of the 
back than was intended, and on account of its weakness would 
not permit a good figure to be given to it. A second mirror 
was cast Jan. 2 6, 1788; but it cracked in cooling. Feb. 16, 
we recast it with particular attention to the shape of the back, 
and it proved to be of a proper degree of strength. Oct. 24, it 
was brought to a pretty good figure and polish, and I observed 
the planet Saturn with it. But not being satisfied, I continued 
to work upon it till Aug. 27, 1789, when it was tried upon the 
fixed stars, and I found it to give a pretty sharp image. Large 
stars were a little affected with scattered light, owing to many 
remaining scratches in the mirror. 
Aug. the 28th, 1789. Having brought the telescope to the 
parallel of Saturn, I discovered a sixth satellite of that planet; 
and also saw the spots upon Saturn, better than I had ever 
seen them before, so that I may date the finishing of the 40- 
feet telescope from that time. 
Description of the Instrument. See Tab. XXIV. to XLII. 
inclusively. 
Fig. 1. represents a view of the telescope in a meridional 
situation, as it appears when seen from a convenient distance 
by a person placed towards the south-west of it. 
The foundation in the ground consists of two concentric 
circular brick walls, the outermost of which is 42 feet in dia- 
