The flexure of small object-glasses by their own weight is of little importance, because every surface is affected alike; but when the aperture is large special precautions have to be taken. The late Mr. Cooke when he had completed the 25-inch object-glass for Mr. Newall’s telescope, introduced a system of counterpoise levers just within the edge which helped to support the object-glass in all positions. Mr. Grubb states that with an aperture of 15 inches, supported on three points, there is decided evidence of flexure, and he proposes, in the 27-inch Vienna refractor, not only to introduce six intermediate supports, thereby following in the footsteps of Mr. Cooke, but with larger apertures to introduce boldly a central support, or to hermetically seal the tube and fill it with compressed air. He has calculated that in the case of an object-glass 40 inches aperture, weighing 600 lbs., two-thirds of its weight could be supported by an air pressure of one-third of a pound to the square inch.
The tube of the telescope when of large size is usually made of iron or wood, and a tube of the latter substance may be made very light and yet sufficiently strong, by wrapping layers of veneer round a central core and fastening the layers firmly with glue. There are generally two or more tubes sliding inside each other at the eye end of the telescope, to carry the eyepiece so as to give plenty of power of adjustment of the length of the tube to suit the different eyepieces, or other instruments used in their place. The tube then is ready to be adapted to any of the mountings to be hereafter considered.
We now come to the mounting of specula, and when we recollect the enormous weights of some of the specimens to which we have referred, it will be obvious that some additional precautions, which are not at all necessary in the case of a refractor, must be taken to insure success.
In reflecting telescopes, the speculum is carried at the bottom of a tube in a sort of tray or cell, which can be adjusted by screws at the back, so as to set the mirror at right angles to the tube, and the conditions of support should be such that the mirror should be as free from strain as if it were floating in mercury. A system of lateral supports in all positions is also necessary.
The action of the telescope depends greatly on the backing of the speculum, and numerous methods of carrying specula on soft backing and systems of levers have been suggested, all aiming at carrying them so that they are free from all possible strain and flexure occasioned by their own weight. For smaller mirrors a soft back of flannel or cloth can be used, and a leather strap placed round the mirror and its back, so as to form the side of a sort of circular tray, will give it sufficient support when inclined to the horizontal. Mr. Browning adopts the plan of making the back of the mirror and its support perfectly flat, so as not to require levers or soft backing; this arrangement would probably fail for mirrors larger than one foot in diameter, although answering admirably for those of less size.
Fig. 70.—Optical part of a Newtonian reflector of ten inches aperture, showing eyepiece, adjusting screws for large speculum, finder, door for uncovering speculum, and counterpoise.
Fig. 71.—Optical part of Melbourne reflector, showing the lattice arrangement for supporting the convex mirror Y, T more solid part of tube fixed to declination axis, W finder.