G4 ARTICLES REFERRED TO IN THE SECRETARY'S REPORT. 
the warmth of the hand for a few seconds, are described and represented. 
From such distortions the speculum would not recover in ten miuutes, and the 
error would be rendered permanent by repolishing in that condition ; and so 
injurious may such causes, even in a lesser degree, prove during the delicate 
process of the final correction of the spherical error, that “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.” 
He found it a matter of not unfrequent occurrence that a speculum would per- 
form much better with rays of a certain amount of obliquity,* deviating from 
2° to 3° from the axis. It is obvious that if this peculiar form could be pro- 
duced at will, and to an adequate degree, it would render the Lemairean or 
front view telescope perfect. Dr. D., however, found that the image was never 
quite as fine as in the usual kind of mirrors. <A letter of Maskelyne subse- 
quently came under his notice, in which he describes a very great improvement 
effected in a 6-foot reflector by Short, by inclining the large speculum 24°, and 
remarks, very reasonably, that “ probably it will be found that this circum- 
stance is by no means peculiar to this telescope ;”’ a hint which may be worthy 
of the consideration of the possessors of reflectors. Such surfaces require to be 
reground, or “re-fined,” ¢.e., finished with the finest emery, to get rid of this 
obliquity, as repolishing, though occasionally successful in a few minutes, wifl 
not always effect it; the attempt failed in one instance, though continued for 
133 hours. 
The modes of forming the requisite tools, of preparing emery, of grinding, 
polishing, testing, (by Foucault’s mode,) and silvering the surface, are somewhat 
too technical to find a place here, but some interesting facts are worthy of being 
referred to. Such is the effect produced by the removal from a cast-iron tool, 
154 inches in diameter, divided into #-inch squares, like a chessboard, of every 
alternate square, by an acid. Though the corrosion extended only to a very 
slight depth, it flattened the curvature of the tool 72 inches. “This shows 
what a state of tension and compression there must be in such a mass, when the 
removal of a film of metal J;th of an inch thick, here and there, from one sur- 
face, causes so great a change.” Another important remark is, how injurious 
an atmospheric disturbance is set up by the intermixture of currents of warm 
air from the observer’s person with the rays falling on or reflected from the 
mirror—an observation which I made many years ago, and which any one may 
test by directing a Newtonian to any bright object, and placing one hand beneath 
the aperture, while an eye-piece held in the other hand, and applied to the eye, 
is carried back a considerable distance, so as to obtain a very long focus, and 
render the ascending currents more visible. It has not, I believe, been generally 
remarked how prejudicial an effect this must have on definition in the front- 
view reflector, and it would be a worthy object of attention to remove the evil 
by the interposition of some non-conducting shield. 
*T became acquainted with this fact many years ago, when working metals for a small 
Newtonian reflector. 
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