90 PROCESS OF POLISHING AND FIGURING 18-IN. GLASS 
that such an effect will continue for some length of time, it having 
been observed to do so in this experiment during 10 m. or more.* 
It may also be noticed that when the source of heat was 
removed it only occupied 2 m. for the glass to return to its normal 
state, and that the thickness of the glass was 2 of an inch. 
The glasses were next removed from the wooden support to an 
iron one of the same shape, and although this had been kept in 
the same room, yet the chill from the iron was enough in less than 
1 m. to produce convexity in the middle surfaces to the extent of 
sohov part of an inch, but also in a regular curve (tig. 11) 
A pressure of 8 Ibs. was next applied to the same central part 
of the glasses, when the colour began to change, but in a different 
manner to that produced by heat 
Two wide bands of colour (fig. 12), appeared, reaching nearly to 
the edge of the glasses and crossed in the mi dle. showing that 
they had become strained in two directions, and their figure com- 
pletely destroyed. 
This shows that although pressure has to be avoided, yet the 
regularity in temperature is the most important factor in the pro- 
duction of the true glass surfaces, and that in finished specula 
(particularly with long foci) a very small inequality in the tem- 
perature of their sides will produce serious defects in their defining 
power. 
It also shows that a material of small heat-conducting power 
would be the most desirable for the mirror to rest upon. 
In conclusion, noting the regular contraction of the lower sur- 
face towards its centre, it has occurred to me that it might be 
possible to employ heat as an agent in the figuring of mirrors, but 
I have not yet tried the effect. 
‘Since this paper was transmitted to the Society I have received 
the last monthly number of the “ English Mechanic,” in which I 
sa to draw attention to two articles. One, No. 25,603, by Mr. 
have, in regard to the practical superiority of employing a mono 
chromatic source of light in the testing of flat surfaces ; and 
another, No. 25,499, by Mr. Wassell (author of a long series of 
valuable papers on Glass Specula), in which my conclusion as to 
the thickness of the silver film seems to be doubted, and is sought 
to be disproved. 
He says: “The fact that a thick film will receive a seratch that 
becomes visible by reflecting light from its sides, proves that the 
silver film is thick enough to perceptibly allow a deviation from 
the figure of the surface below.” 
= Thi ‘ the 
ed Coen ginko lp ge gE Le 
——— consequently these in the lower side are pressed together, and the g 
