388 PROFESSOR W. C. ROBERTS-AUSTEN ON THE DIFFUSION OF METALS. 
distribution of the apparatus which may be used for such measurement of tempera¬ 
ture up to the melting-point of platinum, and a brief enumeration of the several 
parts of the appliance is, therefore, all that need be given here. As shown in 
Plate 7, fig 1, it consists of a camera about five feet long, in which the galvanometer 
is placed. This camera has three doors, and is made separate from the portion of the 
apparatus which contains the moving photographic plate. The two parts are 
connected at J, by flexible leather, the object being to enable the plane of the 
sensitized plate to be adjusted at right angles to the ray of light from the stationary 
galvanometer mirror F. Inside rhe camera is a focussing tube T, containing 
a lens L, which receives external light from the mirror H, and transmits it to the 
galvanometer mirrors F and M. Of these mirrors, M is movable, and is carried by 
the coil of the galvanometer, while F is stationary on an adjustable arm fixed to 
the supports of that instrument, its function being to send a ray of light from the 
mirror H to the slit B, and thus to trace a datum line as the photographic plate 
travels upwards. The temperature is recorded by the variations in the position of 
the spot of light derived from the movable mirror M. There is a screen S to cut off 
light reflected from the brasswork of the galvanometer. The end of the tube T is 
provided with an adjustable brass slit, by means of which the width of the photo¬ 
graphic lines traced on the plate can be varied. The mirror H is mounted on a 
block, which can be adjusted so that external light may be brought from either side. 
The focussing of the lens L may be effected from outside the camera. Plug connec¬ 
tions are provided at the back of the instrument, and the wires a, b, connect the 
galvanometer with the thermo-junctions at the source of heat ; cold junctions being 
interposed, as shown in fig. 4, Plate 7. The photographic plate is secured to its carrying 
slide C by means of little cams, and the carrier C is enclosed in a case K provided 
with a light-tight door, N. The case K is held in position by a pin, P. The con¬ 
nection of the photographic plate with the driving clock is shown at D. The 
sensitized plate moves in front of the slit B, and is lifted by a weight actuating a 
fine clock, constructed by the well-known horologist, Mr. David Glasgow. It may 
be mentioned that the galvanometer stands on three plates, which ensure steadiness 
by providing the well-known combination of the “ hole,” “ slot,” and “ plane.” 
It will be seen on reference to Plate 7, fig. 2, that in the centre of the inner 
cylinder of iron which contains the diffusion tubes there is a central tube of metal L. 
Into this tube several thermo-junctions, JJ'J”, usually three in number, are inserted 
to various depths. Each thermo-junction is, as fig. 2 shows, suitably protected and 
insulated by clay tubes. These clay tubes nearly block up the inner space and 
prevent the free circulation of air, and the four cold junctions with which they 
communicate are shown in fig. 4, Plate 7. These junctions are, by the aid of a 
switch S, fig. 4, actuated by clockwork, placed in turn in communication with the 
galvanometer, and by the deflection of its mirror M (fig. 1) the temperature of any 
given junction at any moment can either be observed on a transparent scale, or recorded 
