698 EAEL OP EOSSE ON THE CONSTEUCTION OE SPECULA OE 6-EEET APEETUEE, 
each revolution of the second eccentric, and therefore there is not the same necessity 
for employing a very thin substratum of pitch ; the process therefore is a much easier one. 
We have found that with the rigid bar, and, indeed, with the jointed one, a slight peri- 
odical movement of the guide D contributes much to free the surface from an annular 
character, for reasons which are obvious. The guide D is mounted now like the eccen- 
tric G, and a band from a small pulley on the axis of B, acting on a large one on the 
axis of D, effects the object in a very simple way. 
When a speculum has been truly ground by the machinery acting with transverse 
strokes, the rigid bar will polish it on very easy terms, and for all the ordinary work of 
tile observatory it will be sufficiently perfect. 
We have long been in the habit of resorting to the rigid bar when out of practice 
and we required at once a fresh speculum. 
A speculum of 3-feet aperture, which has usually been uncovered in all weathers for 
visitors, has frequently been so polished, and it has borne well a quarter-of-an-inch 
lens when tested with a watch-dial while on the engine. When a speculum of 6-feet 
aperture was last polished a rigid bar was employed; and the result was tolerably 
successful. 
Since the publication of Mr. Lassel’s experiments we have several times tried simple 
pitch, the movement being given by the rigid bar, but we have not succeeded in obtain- 
ing as good a surface or as fine definition as when the polisher was prepared in a more 
elaborate manner. 
The combination of soap and ammonia which we employ may be prepared in this way. 
Half a pound of brown soap is dissolved in one quart of warm water, and one quart of 
strong water of ammonia is added. The bottle is then corked and shaken from time to 
time, for a week at least : we think it improves by keeping. One ounce of this mixed 
with eight ounces of water makes the lubricating fluid. The mixture should be made 
the day before, and kept in an open vessel, so that the excess of ammonia may evaporate. 
We were at first apprehensive that in employing this mixture we were endangering the 
hard film of the polisher, and so perhaps sacrificing to some extent truth of surface ; 
but this was found not to be the case unless the ammonia was much in excess. As a 
kind of experimentum crucis, Ave polished specula with simple pitch rather softer than 
usual, employing pure water and the saponaceous mixture alternately, and found that 
the mixture was favourable to truth of surface instead of the reverse. 
We haA'e long ceased to make rouge, as it can be obtained of good quality from the 
rouge-maker. 
In shaping the polisher by applying it to the speculum, we find it better that the 
polisher should be quite cold, while the pitch and resinous composition are slightly 
warm. We pass the flame of a few shavings or of wood-spirit under the polisher with 
its surface doAvn, and instantly apply it to the speculum for half a minute. This is 
repeated till there is satisfactory contact. When the polisher was warm, we found it 
was difficult to avoid compressing the pitch too much. A crane makes the 3-feet polisher 
