1879. ] L. Schwendler—On a new Standard of Light. , ot 
reflection, appears to weaken the light, as was to be expected. Compare 
the results of experiments Nos. 1, 4 and 5. 
7. 
The ratio = of the two lights is independent of the strength of the 
current, which it ought to be. 
These results, although showing nothing extraordinary, 7. e., what 
could not have been foretold without making the experiments, are never- 
theless valuable, since they prove that in the first instance thin glass covers 
take away very little light, and that back-reflection is also very little ; but 
small as these influences are, they have been unerringly measured by the 
Photometer, showing this instrument to be very accurate and the eye 
quite trustworthy. That the light ¢ produced by P. L. S. III was so much 
more intense than 7* produced by P. L. S. II., is due to the fact that the 
Platinum sheet out of which No. II. was cut was much thicker than the 
other. 
Detailed description of the Standard and the method of using it. 
Fig. 3, Plate VIII. gives the construction of the Platinum standard 
in half its natural size. Ineed not give further explanation on this point as 
everything will be readily understood from the drawing. 
Fig. 4 shows the diagram of the connections. 
P. L. S. is the standard— 
G a current indicator, or better, current measurer. The deflecting ring 
must consist of a few convolutions of thick copper wire—of no perceptible 
resistance. The small magnet needle is best to be pivoted, carrying a long 
aluminium index. 
E is the battery, consisting of a few elements of high EH. M. F. and 
low internal resistance connected up successively. Grove’s, Bunsen’s or 
large Daniell’s cells will answer well for the purpose. 
(1) is a stopper by which the circuit can be conveniently opened or 
closed. 
M is a mercury rheostat of about one unit resistance. A groove of 
about 1 mm. section and 1 metre total length is cut into hard wood (not 
ebonite, asmercury does not run well in ebonite). The hard wooden board 
is supported by three levelling screws. 
Further the mercury is iff perfect metallic contact with two iron 
terminals f f. These terminals are not to be fixed to the board, They 
are simply placed into the mercury, which fills small reservoirs at each end 
of the mercury thread. 
The resistance of the mercury rheostat can be easily altered by 
moving the bridge b along the two parallel mercury grooves. If the bridge 
is taken out, the total resistance of the rheostat is in circuit. 
