Nebulæ can be selected so that an insensible gradation shall take place from a coarse cluster like the Pleiades down to a milky nebulosity like that … - William Herschel

In the year 1783 I finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. After two years' observation with it, the great advantage of such apertures appeared so clearly to me that I recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which I wished to undertake, the President of the Royal Society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal bounty.

To conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired whether light be essentially different from radiant heat? In answer to which I would suggest that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one. ...If this be a true account of the solar heat, for the support of which I appeal to my experiments, it remains only for us to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat.

In consequence of this arrangement I began to construct the forty-foot telescope about the latter end of 1785. The woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. In the whole of the apparatus none but common workmen were employed, for I made drawings of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. While the stand of the telescope was preparing, I also began the construction of the great mirror, of which I inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where I then lived, to my present situation at Slough.