254 



HA RD WICKE'S SCIENCE- G OS SIP. 



telescopes. Whichever sort of telescope is employed, 

 as the use of the mirror, or object-glass, is merely to 

 collect light and bring it to one point called the focus, 

 the image there formed must be enlarged. This end 

 is accomplished; by means of a small lens, or com- 

 bination of lenses, known as the eye-piece. In the 

 case of astronomical telescopes the eye-piece usually 

 consists of a tube containing two double convex lenses 

 of different sizes ; the larger of the two faces the object- 

 glass, and is called the field-lens, the other, the eye- 

 lens. These two glasses are so arranged as to be 

 equivalent in use to a single lens, and we can, by a 

 little calculation, find the focal length of the combi- 

 nation. Since the light-gathering power depends on 

 the size of the object-glass or mirror, it is, of course, 

 always the same for a given instrument ; but the 

 amount of magnification depends upon the eye-piece 

 that is used, and is determined by dividing the focal 

 length of the object-glass or mirror by that of the 

 eye-piece. " Deep " eye-pieces are those the focal 

 length of which is short. The first person who 

 actually made a reflecting telescope was Sir Isaac 

 Newton, about 1670. The various forms of reflectors 

 are known as the Newtonian, Gregorian, Casse- 

 grainian, and Herschelian, thus perpetuating the 

 names of their respective inventors. As regards the 

 first three, the rays of light which fall on the mirror 

 are reflected to a smaller polished surface, and thence 

 to the eye-piece, while in the Herschelian the small 

 mirror is dispensed with, the observer's back being 

 turned towards the object to be examined. The 

 making and subsequent grinding of large object- 

 glasses or mirrors are matters of exceeding difficulty, 

 and heavily tax the skill and patience of the artist. 

 Yet the necessary knowledge has so far advanced that 

 the mirrors and object-glasses, having a diameter of 

 six and three feet respectively, which have already 

 been made, will probably be soon surpassed in size. 



Having obtained an insight into the construction 

 of a telescope, we must next consider how it can best 

 be supported so that we may be able to do good 

 work with it. It must be remembered that a some- 

 what inferior telescope on a good stand is far more 

 serviceable than a better instrument on an inferior 

 support. The most simple form of stand for an 

 astronomical telescope is an arrangement in which 

 the main tube is firmly screwed, near its centre of 

 gravity, to a somewhat long piece of metal — a cradle 

 — which is attached by a joint to a vertical pillar. 

 This pillar is fastened to a wooden or metal tripod, 

 but its interior construction is of such a nature as to 

 allow the upper jointed part to revolve at will. 

 With such a stand the telescope will be capable of 

 being moved in a horizontal, as well as in a vertical, 

 direction and, therefore, of being pointed to any part 

 of the heavens we wish. Assuming that we are in 

 possession of a telescope mounted on a stand so firm 

 and steady that it will not allow the instrument to 

 vibrate after being moved, and we are anxious to do 



some real work with it, on the first suitable night we 

 set up our instrument in the garden. In our change- 

 able climate we must not expect to be out very long 

 before clouds come up or rain begins to fall ; every- 

 thing has to be hurriedly brought indoors. Then it 

 is that the observer longs for an observatory, and 

 if his means are commensurate with his zeal, no 

 great length of time will elapse before he is in 

 possession of what is not merely a luxury but an 

 absolute necessity. Then, too, he will bethink him 

 of a better mode of supporting his telescope ; but a 

 consideration of this we will, defer for the 'present. 

 Of observatories there are different sorts and sizes, 

 ranging from tire modest and inexpensive Berthon 

 telescope-house to that magnificent establishment 

 lately completed, to which many an astronomer is 

 directing his eager longings, the Lick Observatory. 



Let us assume that by some fortunate chance we 

 have been able to get within the charmed portals of 

 a well-equipped observatory, and, for the better 

 accomplishment of our design, that we have free 

 ingress and egress at any hour of the day or night, 

 and, in addition, that the Director has the time at his 

 disposal, as well as the inclination, to serve us in 

 every possible way. Our first desire will doubtless 

 be to see the largest instrument in the establishment. 

 Certainly our own little instrument belongs to the 

 same class, but its relationship to the monster now 

 before us seems very distant ; moreover, there is not 

 the faintest similarity between our simple stand 

 and the complicated arrangement by which the 

 enormous tube is supported. Evidently there is a 

 great deal to be learnt here. Our Mentor explains to 

 us how the dome or drum over the instrument is 

 worked. By pulling a rope a slight opening, a few 

 feet wide, is made from horizon to zenith, and by 

 pulling another rope or turning a winch the whole of 

 the dome or drum is caused to revolve. The open 

 part can thus be directed to any part of the sky. 

 Some of these domes are of enormous size, yet they 

 are so well balanced, and their mechanism so nicely 

 made that the apparently cumbrous mass can be 

 easily moved by the expenditure of a very small 

 amount of power. The mounting of the telescope 

 rests on a solid mass of masonry or concrete, 

 extending many feet below the surface of the earth, 

 and entirely disconnected from the floor. This is 

 done to obviate the effects of vibration, than which 

 few things are more annoying to the astronomer. If 

 really good work is to be done, the observer must be 

 at his ease, his hands must be free, he must be able 

 to look straight through his telescope without in- 

 curring a crick of the neck by having to twist himself 

 about into all imaginable and unimaginable positions. 

 We shall find various devices about the room to 

 enable us to accomplish this very desirable end. 

 There are chairs of the ordinary height but with 

 sloping backs, and there are cunning arrangements by 

 which the observer, without leaving his seat, can 



