SPECULA BY HAND, AND EXPERIMENTS WITH FLAT SURFACES. 89 
30° they were found to be broken or displaced at the edge of the 
silver, a distance about 2 x (x= to distance between two suc- 
ceeding bands). 
The thickness of the silver 8 would be thus expressed by » sec. 
5 
face of the glass, the bands were difficult to be observed. The 
above expression might have to be added to any number of half 
wave lengths sec. , but that such was not the case was proved 
by using compound solar light. 
By repeated silvering, or by using a stronger solution, the film 
might be considerably augmented ; yet it may be concluded that 
by ordinary care in the polishing no optical change will be pro- 
duced in the reflecting surface. 
To ascertain the exact effect produced by heat and pressure 
upon these glasses I placed them as before upon a wooden chuck, 
92-inch diameter, and observed a uniform colour at 65° 
I then placed my finger, touching without pressure, upon the 
centre of the top glass and watched the effect. Almost instanta- 
neously the colour changed to regular concentric rings continually 
diminishing in size but increasing in number by additional heat. 
By lowering the point of sight these rings (fig. 10) travelled 
towards the centre, proving the glass to have become concave, and 
that by a measurable quantity. At first glance, this augmenta- 
tion in the film of air immediately below the point to which heat 
was applied might be thought to have been caused by its exces- 
sive expansion in comparison with glass. 
his is however not the case; the glasses were unconstrained, 
and if one part of the air between them expanded, it would only 
edge, B (fig. 10), the very opposite was the result ; both glasses 
expanding approached nearer and nearer to each other until no 
light was reflected. ; 
From this it appears, that when heat is applied at the central 
part of one side of a round piece glass the other side contracts 
‘and becomes hollow in a regular curve from edge to centre, and 
