THE GREAT MELBOURNE TELESCOPE. 
143 
balls, &c., was preserved from the intrusion of either grinding matter or water during 
the subsequent processes of both fine grinding and polishing*. 
The polisher is constructed similarly to that previously used by Mr. Grubb for smaller 
specula. It is formed of a great number of strips of fine deal, 1^ inch wide and 1J 
inch apart, laid in layers and crossed, each layer being firmly glued and nailed to the 
next. The pieces, in the middle of its thickness, are thicker in the centre than at the 
ends, being formed as if they were cut out of a lenticular disk of 4 feet diameter, 1 J; inch 
thick at centre, and f- inch at edge ; on either side of their centre were built, by crossing, 
parallel slips in layers, the outer course at both sides being without intervals. The 
apertures occurring round the edge were stopped up, and the whole turned to the required 
curvature on the face, and strongly varnished. The entire presents the qualities of great 
stiffness, lightness, and freedom from change of figure from either heat or moisture. 
The pitch used for coating the polisher, after being adjusted to the desired hardness, 
was rolled into parallel thickness, cut by pressure into squares while still soft, and these 
applied to the surface of the polishers, using a spirit- or gas-lamp for momentarily sof- 
tening one side of the patch before laying it ; the patches were f-inch squares with 
f-inch intervals, and only one hardness of pitch was used. 
This method of coating a polisher is (for a large-sized one) necessarily tedious, but 
can be done by inexperienced hands, and assuming the proper degree of hardness to 
have been obtained, involves no other uncertainty ; it will also serve for several opera- 
tions. The focus of the first speculum polished was purposely kept within that specified ; 
it turned out to be 30^ feet instead of 32 feet. It was desirable that both large specula 
should be approximately of the same focal length, and this has been accomplished by 
the use of a large spherometer, the difference of focus being about IT inch only, or 
part of the entire.] 
In the process of grinding the pressure is considerable, on an average 112 lbs., and 
the strokes 32 in the minute and of considerable extent ; but for polishing all these are 
much diminished ; the pressure is from 30 to 20 lbs., the strokes are 24 in the minute, 
and a fifth of their former length. The speculum revolves once for 14 strokes ; the 
polisher more slowly, according to the nature of the action. In Mr. W. De La Eue’s 
machine and mine this last motion is effected mechanically, but Mr. Grubb has not 
found this to be essential. 
The small speculum is polished on a similar machine but much smaller ; it is rather 
too large for hand-work, and requires to be specially figured to match the great one, as 
that must be parabolic to be available for photography ; but in this there is no difficulty, 
as the tendency to error is in the direction which the small speculum requires. 
The uncertainty of the polishing process arises from the pitch not being of a proper 
hardness (which depends on the temperature), and the hygrometric state of the air. It 
must always be a very delicate operation, but Mr. Grubb seems to have reduced the 
* Eor some numerical data respecting these operations, see Appendix No. II. 
MDCCCLXIX. 
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