In carrying out a series of photometrical experiments lately, I found that
it was a matter of considerable difficulty to keep the flames of the
standard candles always at their proper distance from the light to be
measured, because the wick was continually changing its position (of
course carrying the flame with it), and thus practically lengthening or
shortening the scale of the photometer, according as the flame was carried
nearer to or farther from the light at the other end of the scale. In
order, therefore, to obtain a correct idea of the extent to which this
variation of the position of the wick might influence the readings of the
photometer scale, I took a continuous number of photographs of the flame
of a candle while it was burning in a room quite free from draught; no
other person being in it during the experiment except a photographer, who
placed sensitive dry plates in a firmly fixed camera, and changed them
after an exposure of 30 seconds. In doing this he was careful to keep
close to the camera, and disturb the air of the room as little as
possible. In front of the candle a plumb-line was suspended, and remained
immovable over its center during the whole operation. The candle was
allowed to get itself into a normal state of burning, and then the wick
was aligned, as shown in the photographs Nos. 1 and 2, after which it was
left to itself.
[Illustration: VARIATION IN PHOTOMETRICAL STANDARDS.]
With these photographs (represented in the cuts) I beg to hand you
full-sized drawings of the scales of a 100 inch Evans and a 60 inch
Letheby photometer, in order to give your readers an opportunity of
estimating for themselves the effect which such variations from the true
distance between the standard light and that to be measured, as shown in
this series of photographs, must exercise on photometrical observations
made by the aid of either of the instruments named.