As an instance, I may mention that my first and most convincing proof of the advantage of the stiller situation was tested by an 18-inch speculum (on which much labour had been bestowed); it was laid aside, but successfully finished after removal, and without undulations or any perceptible defect, and the Observer wrote me, that in good air, he “saw Sirius as a brilliant white dot, without a ray or appendage of any kind, and celestial photographs obtained with it are very fine.” Such results were exceedingly gratifying, as they were obtained with much less labour and uncertainly, and the tedious process of the final touches had not to be repeated so many times.
It is said that the celebrated Alvan Clarke, from the same effects of tremor, never finished an object-glass to his satisfaction above-ground; and Dr. Draper, testing his mirrors at the centre of curvature, to avoid draughts, &c. in an ordinary apartment, resorts to an underground one.
In this little book of “Hints” it may be useful to remind those who possess a speculum of fine quality that they are not produced by a “rule of thumb”—so to speak—and that the difference between _a_ speculum and a really _fine_ one, giving a maximum of defining and illuminating power, is the result of considerable labour and thought, and deserves careful usage.
The Rev. Cooper Key, an amateur of much experience, writes in the “English Mechanic,” that he was eight months (working sometimes eight hours a day), giving his 18 inch speculum its final touches and corrections.
A well known correspondent of the “English Mechanic,” “Hyperion,” tells us he found it impossible to test his 8-1/2 inch mirror in an ordinary room, and had to resort to a tunnel under the clay of his garden. Those who have the means and perseverance to make their own mirror, should be careful not to proceed with the finishing touches until an hour at night when their workshop, if in a town, is free from tremor.
First secure a well-ground and carefully-centred disc, let the polish be as perfect as possible before any attempt is made to figure. Care must be taken that every square or portion of the polisher is of the same consistency and temperature, that the disc may not be acted on irregularly.