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But, unhappily, in proportion to the increase of size in the lens, and consequently of space-penetrating power, is the diminution of light from the object, by diffusion of its rays. And for this evil there is no remedy within human ability; for an object is seen by means of that light alone which proceeds from itself, whether direct or reflected.
From a star-gazing musician he was at once transformed into an eminent astronomer; he was relieved from the drudgery of a toilsome profession, and installed as Royal Astronomer, with a modest salary of £200 a year; funds were provided for the construction of the forty-foot reflector, from the great space-penetrating power of which he expected unheard-of revelations; in fine, his future work was not only rendered possible, but it was stamped as authoritative.
His ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "space-penetrating power" by increasing their light-gathering surface. These, as he was the first to explain, are in a constant proportion one to the other.
For a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. In other words, it will possess double the space-penetrating power of the smaller instrument.
This lacuna Herschel tried to fill up in 1799; such was the aim of the memoir entitled, On the space-penetrating Power of Telescopes. This memoir contains excellent things; still, it is far from exhausting the subject. The author, for instance, entirely overlooks the observations made by day.
Sir William Herschel had determined the places of 2,500 nebulæ in the northern hemisphere; they were examined by his son, and drawings made of some of the most remarkable, but when these nebulæ were viewed through Lord Rosse's telescope, they presented a very different appearance, showing that the apparent form of the nebulæ depends upon the space-penetrating power of the telescope, a circumstance of vital importance in observing the changes which time may produce on these wonderful objects.
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