We may now turn to the four-foot silver-on-glass Newtonian now in course of completion at the Paris Observatory.
The illustration which we give represents the telescope in a position for observation. The wheeled hut under which it usually stands, a sort of waggon seven metres high by nine long and five broad, is pushed back towards the north along double rails. The observing staircase has been fitted to a second system of rails, which permits it to circulate all round the foot of the telescope, at the same time that it can turn upon itself, for the purpose of placing the observer, standing either on the steps or on the upper balcony, within reach of the eyepiece. This eyepiece itself may be turned round the end of the telescope into whatever position is most easily accessible to the observer.
Fig. 145.—Great Silver-on-Glass Reflector at the Paris Observatory.
The tube of the telescope, 7·30 metres in length, consists of a central cylinder, to the extremities of which are fastened two tubes three metres long, consisting of four rings of wrought-iron holding together twelve longitudinal bars also of iron. The whole is lined with small sheets of steel plate. The total weight is about 2,400 kilogrammes. At the lower extremity is fixed the cell which holds the mirror; at the other end a circle, movable on the open mouth of the telescope, carries at its centre a plane mirror, which throws to the side the cone of rays reflected by the great mirror.
The weight of the mirror in its barrel is about 800 kilogrammes; the eyepiece and its accessories have the same weight.[[17]]
It will be quite clear from what has been said that the manipulation of these large telescopes at present entails much manual and even bodily labour, and when we come in future to consider the winding of the clock, the turning of the dome, and the adjustment of the observing chair, it will be seen that the labour is enormous. To save this, in all the best instruments everything is brought to the eye-end of the telescope, movements both in right ascension and declination, reading of circles, and adjustment of illumination. Mr. Grubb has suggested that everything should be brought to this point, and that, by the employment of hydraulic power, “the observer, without moving from his chair, might, by simply pressing one or other of a few electrical buttons, cause the telescope to move round in right ascension, or declination, the dome to revolve, the shutters to open, and the clock to be wound.” He very properly adds, “This is no mere Utopian idea. Such things are done, and in common use in many of our great engineering establishments, and it is only in the application that there would be any difficulty encountered.”
The Driving Clock.
In a previous chapter it was stated that in all large telescopes used for the astronomy of position, whether a transit circle or the alt-azimuth, what we wanted to do was to note the transit of the star across the field—the transit due to the motion of the earth; but that when we deal with other phenomena, such, for instance, as those a large equatorial is capable of bringing before us, we no longer want these objects to traverse the field, we want to keep them, if possible, absolutely immovable in the field of view of the eyepiece, so that we may examine them and measure them, and do what we please with them.
Hence it was that we found driving clocks applied to equatorials; and our description would not be complete did we fail to explain the general principles of their construction. They are instruments for counteracting the motion of the earth by supplying an exactly equal motion to the tube of the telescope in an opposite direction.