The Wonders of Optics

by Fulgence Marion

Whilst experimenting on the Gregorian telescope, Newton made certain improvements in its construction, which we shall proceed to describe. A glance at [fig. 46] will show that the path of the rays is much more simple than in the instrument we have just noticed.

Fig. 46.—Section of a Newtonian Telescope.

The rays of light A B are first reflected from the concave mirror M on to the surface of the small plane mirror m, which is placed at an angle of 45°, and reflects them as far as the point A´ B´, where they form the image to be magnified by the eye-glass. It is therefore at the side of the instrument, and not at the end, as hitherto, that the observer is placed, and at right angles to the path of the rays. Observers looking at an object through a Newtonian telescope for the first time are generally sufficiently astonished to find that there is really no difficulty after all in seeing round a corner. We shall presently return to the subject of Newtonian telescopes, which were abandoned by astronomers for many years, until they were brought into use again by M. Foucault, a distinguished French philosopher.

Towards the end of the last century Sir William Herschel invented and constructed the reflecting telescope which bears his name. His great object was to avoid the loss of light consequent on the double reflection which took place in all instruments constructed up to that time, and he succeeded at last in making a telescope in which the observer looked directly through the eye-piece at the image formed by the mirror, which was inclined in such a manner that the rays were reflected to the lower edge of the open end of the tube. In using this kind of telescope the observer is placed with his back to the object he wishes to examine, a position that is even more astonishing to those unaccustomed to the use of a Herschellian telescope than the one assumed when employing an instrument of the Newtonian construction. This position has the defect of causing a small portion of the rays proceeding from the object to be intercepted by the head of the observer, but the amount of light lost is so small in comparison to the size of the mirror that in practice it amounts to nothing.

The dimensions of the telescope constructed by Herschel were enormous for that day. It measured 40 feet long, and the mirror was 4 feet in diameter. It was supported by a complicated system of scaffolding, pulleys, and cords, and was capable of magnifying an object 6,000 times. It was by means of this splendid instrument that Sir William Herschel made those wonderful discoveries in astronomy which are inseparably associated with his name. With it he discovered the planet Uranus, many of the double stars, and a large number of nebulæ, which up to that time were unknown. His son, Sir John Herschel, inherits his father’s talents as an astronomer, and has enriched science with numberless observations and discoveries of the greatest importance made with this fine instrument. [Fig. 47] shows the construction of the Herschellian telescope, and the path of the rays may be easily followed by the student without any help from us.

The vulgar, ever prone to make mountains out of molehills, magnified the power of Sir William Herschel’s telescope beyond all bounds. Stories were circulated about his having given a dinner in the interior of the tube to a select party of friends, but as the diameter of the telescope was only a little more than 4 feet, the entertainment, to say the least of it, would have proved somewhat inconvenient to the guests. Another story, which was credited by great numbers of people, was that he had discovered inhabitants in the moon, but that he hesitated to make the matter public for fear he should be prosecuted for spreading atheistical notions. In fact, the tales told of Sir William Herschel’s telescope were endless, and caused the astronomer great inconvenience by attracting crowds of idle people to the neighbourhood of Slough, where he vainly endeavoured to carry on his investigations in peace and quietness. It was in vain that these silly assertions were disproved again and again. Having once believed them, people were slow to reject them, and the story of the dinner was told over and over again for many years.

Fig. 47.—The Herschellian Telescope.

The instrument above described is one of those known as front view telescopes, on account of the image of the star being reflected from the surface of the mirror, which was placed obliquely at the bottom of the tube in front of the observer, who examined it by means of the eye-piece without any other reflection taking place, thereby effecting a saving of light, which fully compensated for any loss caused by the mirror being placed askew. The concave mirror made by Herschel alone weighed a ton, to say nothing of the enormous tube and its fittings. Herschel had consequently to invent a special apparatus for holding and moving this gigantic instrument. The moving gear consisted of a mass of beams, pulleys and cords, reminding one more of the rigging of a ship than of a philosophical instrument. The apparatus for moving the telescope appeared so complicated to the casual observer, although in reality it was very simple, that it doubtless contributed in no small degree to the propagation of the fanciful stories we have already spoken of.