A SILVERED GLASS TELESCOPE. 9 



disturb a surface injuriously. Frequently mirrors in the process for correction of 

 spherical aberration wiU change the quality of their images without any perceptible 

 reason for the alteration. A current of cold or warm air, a gleam of sunlight, the 

 close approach of some person, an unguarded touch, the application of cold water 

 injudiciously will ruin the labor of days. The avoidance of these and similar causes 

 requires personal experience, and the amateur can only be advised to use too much 

 caution rather than too little. 



Such accidents, too, teach a useful lesson in the management of a large telescope, 

 never, for instance, to leave one-half the mirror or lens exposed to radiate into cold 

 space, while the other half is covered by a comparatively warm dome. Under the 

 head of the Sun-Camera, some further facts of this kind may be found. 



Oblique Mirrors. — Still another propensity of glass and speculum metal must be 

 noted. A truly spherical concave can only give an image free from distortion Avhen 

 it is so set that its optical axis points to the object and returns the image directly 

 back towards it. But I have polished a large number of mirrors in which an image 

 free from distortion was produced only when oblique pencils feU on the mirror, and 

 the image was returned along a line forming an angle of .from 2 to 3 degrees with 

 the direction of the object. Such mirrors, though exactly suited for the Herschelian 

 construction, will not officiate in a Newtonian unless the diagonal mirror be put 

 enough out of centre in the tube, to compensate for the figure of the mirror. Some 

 of the best photographs of the moon that have been produced in the observatory, 

 were made when the diagonal mirror was 6 inches out of centre in the 16 inch 

 tube. Of course the large mirror below was not perpendicular to the axis of the 

 tube, but was inclined 2° 32'. The figure of such a concave might be explained by 

 the supposition that it was as if cut out of a parabolic surface of twice the diameter, 

 so that the vertex should be on the edge. But if the mirror was turned 180° it 

 apparently did just as Avell as in the first position, the image of a round object being 

 neither oval nor elliptical, and without wings. The image, however, is never quite 

 as fine as in the usual kind of mirrors. The true explanation seems rather to be 

 that the radius of curvature is greater along one of the diameters than along that 

 at right angles. How it is possible for such a figure to arise during grinding and 

 polishing is not easy to understand, unless it be granted that glass yields more to 

 heat and compression in one direction than another. 



After these facts had been laboriously ascertained, and the method of using such 

 otherwise valueless mirrors put in practice as above stated, chance brought a letter 

 of Maskelyne to my notice. He says, " I hit upon an extraordinary experiment 



Avhich greatly improved the performance of the six-feet reflector" It was 



one made by Short. " As a like management may improve many other telescopes, 

 I shall here relate it : I removed the great speculum from the position it ought to 

 hold perpendicular to the axis of the tube when the telescope is said to be rightly 

 adjusted, to one a little inclined to the same and found a certain inclination of about 

 2|° (as I found by the alteration of objects in the finer one of DoUond's best night 

 glasses with a field of 6°), which caused the telescope to show the object (a printed 

 paper) incomparably better than before ; insomuch that I could read many of the 

 words which before I could make nothing at aU of It is plain, therefore, that this 



2 May, 1864. 



