694 EAEL or EOSSE ON THE CONSTEUCTION OF SPECULA OF 6-FEET APEETUEE, 
The principle of Mr. Whitwoeth’s method may obviously be carried out, with glass 
or speculum-metal, by employing small laps and grinding-powders instead of the scraper ; 
but as a scraped surface consists of a maze of curves of varying flexure, a surface ground 
in detail must always, I should think, in some degree partake of the same character, 
and, though it may not anywhere deviate much in general outline from the required 
form, minute deviations must exist in every part. M. Foucault seems to have been 
successful in improving surfaces of moderate dimensions, by his ingenious process of 
testing and polishing in detail ; how far such a process will succeed in improving large 
surfaces which have been in the first instance properly wrought, has not, as far as I am 
aware of, been ascertained. Our practice always has been to repolish when the surface, 
tested by the method described in the ‘Transactions’ for 1840, has proved to be de- 
fective. If a few glaring defects are at once seen, the whole surface is always faulty, 
though in a less degree. 
The only change we have made in the polishing-machinery consists in substituting an 
elliptic for a circular wheel in driving the second eccentric. The major axis is at right 
angles to the throw of the eccentric, and is to the minor axis as three to one. The band 
is merely a rope working in a deep groove ; and a straining pulley, freely acted upon by 
a weight, secures the necessary tightness in all positions of the ellipse. The obvious 
effect of this arrangement is to diminish the time the polisher overhangs the speculum, 
and so to reduce, to some extent at least, a source of error. We now employ in every 
case a separate tool for grinding and polishing, which is a great convenience, especially 
as we always regrind the speculum after it has been brought in from the telescope. 
There seems to be no doubt that in some cases considerable change of figure had taken 
place. The grinding-tool, when true, will be bronzed all over, and the speculum, when 
examined in every position as to light, will appear uniform. 
We still consider the process of polishing described in detail in the ‘Transactions’ for 
1840 as the best, with this addition, that we employ a combination of brown soap and 
ammonia, instead of pure water, during the latter part of the operation. We had then 
tried this lubricating mixture but too recently to feel justified in recommending it. 
The great objection, however, to the whole process is the difficulty of carrying it out. 
I have had communications from time to time from many persons in whose hands it has 
failed ; and I am not surprised ; for although everything has usually gone on smoothly 
when we were in the midst of experiments and in constant practice, yet after the lapse 
of even one year, when we have had occasion to repolish a speculum, there have been 
often disappointments. The difficulty arises from the necessity of employing two strata 
of resinous matter, one so hard, and both so thin. If in preparing the polisher the 
hard resinous composition is suffered anywhere to come in contact with the iron, the 
polisher will not retain its figure, and there will be a failure. A small chip of wood in 
the pitch will produce the same effect. If the water or lubricating mixture is supplied 
too sparingly, the polisher will begin to dry in spots, the rouge and abraded matter will 
collect there, and the thin stratum of pitch will be compressed till the accumulated 
