140 Proceedings of Royal Society of Edinburgh. [sess. 
After the receiver is completed, with its pipes, &c., it is graduated. 
This is done by filling it with water, which is easily accomplished 
by inverting it and pouring in water through 0, while the air is 
allowed to escape by T. The receiver is then put in its correct posi- 
tion and levelled; after which 350 c.c. of water is run out, and 
three marks are made at equal distances round the cylinder at the 
water-level. In using the receiver afterwards the water is kept at 
this level. 
The receiver is supported at a height of 13 inches above the 
table. For this purpose three sockets, W, W, W, figs. 1 and 4, 
may be cast on the bottom plate C, into which three legs may be 
fixed ; or the tripod stand shown in section and plan in fig. 5 may 
he used. In the tripod sketched, in addition to the three sockets for 
the legs, there is a fourth for carrying a rod to support the lens 
used for condensing the light on the stage. This saves a special 
stand for the lens. The tripod is drawn to a scale of \ size. 
On Plate III. fig. 7, is given a full size sketch of the counting 
stage, and the top end of the tube D. This stage is made of 
silver and highly polished. It is ruled with lines at right angles 
to each other, and at exactly 1 mm. apart. These lines should be 
extremely fine. To the under side of the stage is fixed a rod which 
is used for holding it when polishing. This rod also fits into 
the socket in the top of the tube D and keeps the stage in its 
place. 
As the silver counting stage requires to be kept well polished, it 
is necessary to remove it frequently from the receiver. In actual 
practice I generally take it out after the day’s work. To enable 
this to be done easily, there are two plans, either of which may be 
adopted. The plate B, forming the top of the receiver, may be 
fitted on with grease. As the two surfaces are ground, it makes an 
easily opened air-tight joint. If this plan is adopted, then the tube 
D may be firmly fixed to the bottom of the receiver ; but it should 
be retained, as it has other duties besides that of supporting the 
stage. The other plan is the one shown in the drawings. The top 
of the receiver is cemented on, and the stage is withdrawn from the 
receiver by taking out the tube D to which it is attached, the 
indiarubber tube H making the joint between the tubes D and F 
air-tight. The principal objection to the first plan is that it is very 
