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high pair of steps. With a further slight modification two persons can view the same object at once.

The adjustments of a modified Cassegrainian are rather more difficult than those of the ordinary form, on account of additional reflective surface. A Cassegrainian mirror should always have a central hole, if made of speculum metal; or if glass, a central spot may be cleaned off the silver film. First adjust the convex mirror by altering the three screws and central clamping screw-pin. To do this properly, remove the flat mirror from the tube, and looking through the central hole in the large mirror, alter the screws until as much as possible of the reflective surface of the large mirror is seen in the centre of the small. If this reflection cannot be procured as an entire circle, adjust the three screws which move the large speculum until it resembles Fig. 15. The adjustments of the convex and great specula must be perfect before the flat should be replaced. When everything appears correct, place the flat in position and adjust it through the door or opening in the same way as the flat is adjusted in the Newtonian telescope. If preferable, the flat can be adjusted at the outset. To do this correctly, cover the entire surface of the large mirror with a sheet of white paper, and place the flat so that it reflects to the adjusting-piece, the open end of the tube with the central disc. Next adjust the convex mirror so that it appears as a complete white disc when viewed on the flat. When these two adjustments are perfect, remove the paper, and so adjust the large speculum that its complete reflection is seen on the convex mirror, with the dark spot exactly in the centre. If the latter plan be employed, the effects of all the adjustments are to be viewed from the adjusting-piece, instead of from the centre of the large mirror. All the adjustments of a Cassegrainian must be very exact, in order that good definition may be obtained.

THE HERSCHELIAN TELESCOPE.

No small mirror being required in this form, the whole of the central rays are employed, whereas in all other forms of reflectors they are intercepted by the small mirrors. But it seldom happens that a concave mirror can be tilted, and therefore receive incidental rays without affecting its definition. The diffraction rings round a star would be oval, and any object viewed would be slightly distorted. It will be obvious that the distortion will increase as the focus is shortened, and therefore the focus of a Herschelian telescope should

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