86 PROCESS OF POLISHING AND FIGURING 18-IN. GLASS 
Figures 4, 5, and 6 (plate 1) represent the corresponding 
guring forms of the polisher ; by increasing or diminishing the 
acting part of these surfaces the time required can be altered. Of 
course the same object may be obtained by reducing the squares, 
as long as the given proportion is maintained. 
Pitch being a yielding (non-elastic) substance, might be expected 
not to act similarly to a rigid surface, but still I have found Nos. 
1 and 2 to give the desired result. No. 3 does not seem to answer 
with the weight of the glass over the polisher. 
The form with which the present glass was figured was No. 2. 
Both polisher and glass having been regularly raised in tempera- 
ture were left together (the glass having been now and then slightly 
moved round its axis) until cool, after which the usual stroke for 
keeping the spherical form was proceeded with for about ten 
minutes, when the correction was found sufficient. 
In another case, No. 1 form was used upon a similar mirror, 
but with only 10’ 4” focus requiring abrasion at the edge exceeding 
revolved. By this motion rings were expected to appear, but such 
was not the case, and in less than 10 minutes an over-corrected 
but true surface was the result. 
The greatest inconvenience in this method is, that should the 
mirror become over corrected, or a hyperboloid, the polisher must 
be remodelled before the spherical form can be restored. +A pet 
fectly even temperature must exist, and the polishing powder be 
centre of curvature, first invented by M. Foucault. For use ™ 
this test the fourth and fifth columns of the above table have been 
calculated, but it was shown that in mirrors in which the length 
the spherical form was required, and that the amount of such cor- 
rection would increase in the ratio 3, (y constant), from which it 
Such is, however, not the case except within certain limits. 
The rate of decrease in the amount of correction required 1s very 
rapid with the increase of radius of curvature, but the injuriovs 
