_Refractors and Reflectors._—The relative merits of refractors and reflectors[7] have been so frequently compared and discussed that we have no desire to re-open the question here. These comparisons have been rarely free from bias, or sufficiently complete to afford really conclusive evidence either way. There is no doubt that each form of instrument possesses its special advantages: aperture for aperture the refractor is acknowledged to be superior in light-grasping power, but the ratio given by different observers is not quite concordant. A silver-on-glass mirror of 8-inches aperture is certainly equal to a 7-inch objective in this respect, while as regards dividing power and the definition of planetary markings, the reflector is equal to a refractor of the same aperture. The much shorter focal length of the reflector is an advantage not to be overlooked. A century ago Sir W. Herschel figured his specula to foci of more than a foot to every inch of aperture, except in the case of his largest instruments. Thus he made specula of 18½-inches and 24-inches diameter, the former of which had a focal length of 20 feet and the latter of 25 feet. The glass mirrors of the present time are much shorter, and the change has not proved incompatible with excellent performance. Calver has made two good mirrors of 17-1/4-inches aperture, and only 8 ft. 4 in. focus. Mr. Common’s 5-foot mirror is only 27½ feet, so that in these instances the length of the tube is less than six times the diameter.
It has long been proved that refractors and reflectors alike are, in good hands, capable of producing equally good results; and we may depend upon it that, in spite of all argument and experiment, both kinds of telescope will continue to hold their own until superseded by a new combination, which hardly seems likely. If the observer is free from prejudice, he will have no cause to deplore the character of his instrument, always supposing it to be by a good maker. Be it object-glass or speculum, he will rarely find it lacking in effectiveness. It happens only too often that the telescope or the atmosphere is hastily blamed when the fault rests with the observer himself. Let him be persistent in waiting opportunities, and let the instrument be nicely adjusted and in good condition, and in the great majority of cases it will perform all that can reasonably be expected of it.