SPECULA BY HAND, AND EXPERIMENTS WITH FLAT SURFACES. 8] 
Draper, Mr. Wassell (English Mechanic), and others, until no 
emery holes or scratches could be seen by looking in an oblique 
direction at ing eflected image of the sun near the foc 
In all cases, lances, that I have seen described, ‘the sang 
polisher ae wan directed to be graduated into squares as in fig. 2 
where it will be seen that the circles described from the centre 
will all intersect the squares at nearly the same angle. have 
read that Lord Rosse, Dr. Draper, and others, had to add an extra 
would naturally produce such, if the thats continually movell i in 
the same track. I also found that when the pitch was very hard 
the glass would move more easily along the grooves than in any 
other direction ; and therefore I graduated mine as shown (fig. 3), 
where it will be found that these defects are obliterated, and 1 may 
state that I have, without side motion, polished (by hand) for hours 
continually without producing as much as a trace of a ring. 
The polishing for this size of speculum (by hand) sesso a 
about seven or eight hours, but in the one before us ight it was 
accomplished in five hours thir ty minutes ; after w hich it was parse 
and found to have retained its original shape, slightly inclined to 
the aici (This, however, may have been changed ree 
.during the polishing.) 
shown hereafter in my method of figuring, or in convertin 
the cette into a paraboloid of revolution, the spheroidal an 
would be as good to start from as spherical ; but it must not be 
Foucault test of a true spherical surface at the centre of curvature, 
that variations of 0000001 inch (as is proved hereafter) can be 
noticed. Supposing, however, that a true spherical surface has 
_ obtained, the most delicate part of the process commences, 
to change it into a paraboloid of revolution, or what is the 
same, to correct the spherical aberration 
Some opticians have attained great success in this operation, and 
ere can be no doubt that a similar effect can be produced more 
or ae Socicakiy in several ways. I will here enumerate some of 
them 
the centre seems to me dou btful 
2 26 gredaily lengthening the strokes (Dr. Draper), not 
ried, 
