442 
DR. W. MANSERGH VARLEY ON THE PHOTO-ELECTRIC 
the photo-electric currents were measured remained invariable, whilst in the other the 
conditions were altered as required. The first apparatus served as a means of testing 
the regularity of the intensity of the light, and, on the assumption that the photo-electric 
effect is proportional to the intensity, of correcting for any small irregularities 
which might occur in the strength of the light, though this was rarely necessary, 
while in the second apparatus the 
experiments proper were carried out. 
The apparatus with which most of 
tlie series of observations were made is 
shown diagrammatically in fig. 2. 
The luass vessel, BB, consisted of 
two cylinders of different diameters 
arranged end-on as shown. An opening 
4’5 centims. in diameter, cut in the 
front face of the larger cylinder, wms 
closed by a quartz window Q, attached 
by sealing wmx. Two brass side tubes 
were soldered on to this cylinder, 
through one of which passed a glass 
Fig. 2. tube G. Through this was fixed a 
stout brass rod which siq3j)orted a thin 
copper tube T, one end of which was covered l)y a veiy fine copper grating C, the 
wires of which were 0'25 millim. in diameter, placed 0'25 millim. apart, and which 
formed one electrode. The other side tube served to connect to a pump and gauge. 
Into the back of the smaller cylinder fitted a wide glass tube H, drawn down 
narrower at the other end, into which a second glass tube, J, would just slide easily. 
A brass rod supporting the other electrode, F, passed through this inner tube, and 
by this arrangement the electrode could be changed or removed to he polished by 
breaking the one joint between the wide glass tube H and the brass cylinder. 
All the joints SS w^ere of sealing wax and perfectly air-tight. They were 
arranged as far as possible so as to ensure the insulation of the electrodes re7naining 
permanently good. 
The joint between the two glass tubes carrying the rod connecting to the electrode F 
(that at li) was made by slipping rubber compression tubing over the two tubes, after 
lubricating 'well with glycerine. This made a perfectly air-tight joint and enabled the 
distance betw^een the two electrodes to l)e altered by any desired amount, when the 
apparatus was exhausted, without admitting any air and thereby altering the 
pressure. 
The front of the tube T, covered by the gauze, or rather grating, formed of 
similar wires to those described above, only allowed the light to fall on the central 
portion of the electrode F, and the apparatus had the further advantage that 
