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air currents can be obviated by making the tube considerably larger than the speculum, for it was found that the currents do not revolve in the centre of the tube but just within its circumference.

We have lately made all our large reflectors with open ventilated tubes, for if the temperature of the air is the same inside and outside the tube, no air currents occur, and instead of having to wait half-an-hour, steady definition is at once obtained, and is not affected by change of temperature.

There are several advantages possessed by reflectors over refractors. In respect to definition, a more perfect mirror can'be worked than an object-glass, it being impossible to quite correct chromatic aberration. A well figured speculum will therefore give the finest possible definition of those celestial objects which appear coloured ; such beautiful double stars as j3 Cygni, y Andromedse, r} Cassiopese, a Herculis, &c., and the delicate tints on Mars, Jupiter, and Saturn being perfectly shown. A most important superiority of reflecting telescopes is their cheapness. In the construction of an object-glass many difficulties arise never experienced by the maker of a mirror. Each glass must be of perfectly uniform density and ascertained refractive index; from these data an elaborate mathematical process shows what curves are necessary, and then comes the grinding and polishing of at least four surfaces, and lastly, the edging and correct centering of the worked lenses. It is quite true that only spherical curves have to be worked, but the final correction often necessitates re-working one or more surfaces several times.

A great advantage of the Newtonian, and especially of the Casse-grainian form as modified by Nasmyth, consists in the easy position of the observer. When viewing zenith objects with a refractor, the' observer’s head must be thrown so far back that continuous observation is most unpleasant, unless a diagonal be employed, and this is often uncomfortable to work with, and occasions loss of light. But with the above-mentioned reflectors the eye-piece can be always horizontal, and therefore the head and neck are always comfortable.

The last advantage of reflectors that need be mentioned is their very compact nature, ensuring thereby great steadiness and facility of employment, and requiring a smaller observatory.

Until quite lately, all varieties of reflecting telescopes were fitted with metallic mirrors. These were open to very serious objections, amongst them being the extreme difficulty of forming a metallic alloy capable of being highly polished, and remaining brilliant in spite of atmospheric and other influences. Should the surface of a metallic mirror once become tarnished, it must be repolished—an operation of the greatest difficulty, on account of the extreme danger of the speculum losing its original parabolic curve ; and thus many a well corrected mirror may be rendered perfectly useless in attempting to remove a stain. This difficulty has been entirely overcome in glass mirrors—by coating the parabolic surface with a very thin film of pure silver about the same thickness as gold leaf.

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