534 
Proceedings of the Royal Society of Edinburgh. [Sess. 
represents the arrangement. A copper casting was made somewhat like a 
saucepan in shape, the inside being shaped cylindrically, of depth 6 cm. 
and of internal diameter 8 cm. There was a circular hole in the bottom 
of the hollow. The one beam of light was reflected by the mirror M x , 
Fig. 2. 
passed vertically downwards through the hole, was reflected by the mirror 
M 2 and arrived at the upper slit S x ; the other beam was reflected by the 
mirrors N x , N 2 , and entered the lower slit S 2 . Two moulded glass cells with 
plane bottoms were fixed one inside the other, as shown in the figure, with 
their bases parallel and so that the distance between them could be varied. 
The liquid investigated was contained between the two cells. There was a 
lid for the cylinder, not shown in the diagram, with a circular hole for 
admitting the light from the mirror M r Water or amyl alcohol was boiled 
in the tin and condensed in the condenser above, the heat flowed along the 
copper and thus the liquid was maintained at a constant temperature. 
