This will occur especially on frosty nights after a warm fine day. When the telescope is in the open air, ice will not infrequently form on the top of the tube, and the effect will be at once obvious in the irregularity of a star image at the edge corresponding to the top of the tube. The use of a small diaphragm will at once obviate this defect and give perfect images. On such nights a wooden tube is preferable to a metal one. It has been frequently my experience, when finally testing an instrument, that when I have been dissatisfied with its performance, and in order to trace the cause of the apparent defect, have removed the mirrors to my wooden testing tube, I have found every defect removed, and imperfect replaced by perfect images. The explanation is simple. Not only is the wooden tube much larger in diameter than the specula, but wood is of itself less sensible than metal to differences of temperature.

To make the tubes some 2 or 3 inches larger in diameter than the speculum would cure this evil, but it would involve various inconveniences and expenses in structure, which would more than outweigh the advantage of overcoming a hindrance which is after all only temporary and occasional.

The experiment of perforating or ventilating tubes has now been thoroughly and exhaustively tried, with the result that there is a fairly general consensus of experienced opinion against their use. The advantages are merely theoretical; the disadvantages are grave. Among them are the admission of cross reflections in every direction, the admission of dust, imperfect protection of the mirrors, and a decided loss of strength and rigidity to the tube.

CATALOGUE
OF
Silvered Glass Reflecting Telescopes, &c.,
AND THEIR ACCESSORIES,

OCTOBER, 1880.

G. Calver, while introducing the list of prices below, has the greatest confidence in calling the attention of the practical astronomer and the amateur to the very moderate prices charged, considering the principle and style of the mounting, and the firm and well-fitted arrangements in all the parts.

The principle of the mounting is that which is the most convenient to use, and with the greatest degree of steadiness with the easiest movements. Fig. 1 is the best mounting ever applied to the Reflecting Telescope.

He is also able to state that, with his processes and special facilities for working large specula, and substantially mounting them, he is prepared to construct automatic equatorials of large sizes, and with any special arrangements that may be required.

Fig. 1 is a very excellent mounting, and admirably suited for large instruments, especially when clock-power is applied; and as now manufactured by G. Calver, is the most complete and reliable, its details of construction being such as to give the greatest freedom of motion and steadiness. For large sizes the top of polar axis works on friction rollers; and, as a superior German stand, its arrangements and means of astronomical as well as optical adjustments, are such as to make it an instrument of precision.

To ensure these qualities many well considered arrangements—entailing careful and expensive workmanship—must be provided; details that do not easily admit of description in an ordinary catalogue, or can be shown in an engraving, but are duly appreciated by the observer, who will find their value by practice.