250 Dr. H. Draper on the Photographic Use of 



judging of faults, and the hand quick at correcting them, a 

 short time suffices to bring the glass concave to a state of per- 

 fection. If the rough grinding be completed, and the convex 

 tool put into good condition, a single day should finish the 

 surface. 



In the beginning of the experiments, one of the most serious 

 difficulties encountered was that arising from the irregular dis- 

 tribution of heat through the mass of the disk of glass. When, 

 for instance, a mirror of 15 J inches is polished by a pitch tool 

 of the same size, it is hard to avoid the production of rings of 

 varying focal length, owing to the overplus of heating towards 

 the middle. Where the tests are applied at the centre of cur- 

 vature, and the operator does not have to depend on indications 

 derived from telescopic observation, these and similar imperfec- 

 tions are much more easily detected and avoided than where 

 they have to be disentangled from atmospheric disturbances. 



Another obstacle, which proved to be formidable at first, was 

 the unequal amount of compression that disks of glass and spe- 

 culum-metal suffer when supported on different parts of their 

 edges. In all the large disks examined, there has been a dia- 

 meter of minimum compressibility, which ought to be kept 

 always vertical. If set horizontally, the mirror immediately 

 gives double images. 



As regards the best machine for producing parabolic surfaces, 

 the experience of six years on several different ones, including 

 those of Lord Rosse and Mr. Lassell, has brought me to the 

 conclusion that the most satisfactory results are to be obtained 

 only by employing polishers of much less diameter than the 

 surface to be produced— a method of working first published by 

 M. Foucault. A better mirror can be made by small local 

 polishers moved by the hand, than by a full-sized polisher moved 

 by any machine that I have tried. This is due to the complete 

 control that the operator gains over the distribution of heat and 

 moisture, and to the power of rubbing away only those parts 

 which are necessary to be removed in order to extricate the 

 parabolic surface below. The method resembles that of scra- 

 ping used by mechanics to produce true planes. At the same 

 time a thorough knowledge of the appearance of spherical, ellip- 

 tical, parabolic, hyperbolic, and other surfaces at the centre of 

 curvature must be gained. I have also polished several mirrors 

 which could only bring oblique pencils to a sharp focus, and 

 which were suitable for the Herschelian construction, the amount 

 of the obliquity, 2° 30', just carrying the image to the edge of 

 the tube. Some of the best lunar photographs were taken when 

 the diagonal mirror of the Newtonian was 6 inches out of cen- 

 tre in the 16-inch tube. 



