140 
PROFESSOR A. M. WORTHINGTON AND MR. R. S. COLE 
slieet of thin tinfoil rubbed suiooth over the inside, and renewed occasionally after it 
had become too much dimmed by magnesium oxide. This mirror, with the sparking 
wires close in front of it, was brought within 8 or 9 centims. of the place of impact, 
and, in order to brinsf out the details of the confio-uration to the o-reatest advantao;e, 
the beam of Imht was directed down on the surface at an angle of between 30° and 
45° with the horizontal, and the camera was so placed that the line of sight was at 
right angles to the plane of incidence of the axis of the illuminating beam, and also 
inclined at about 30° to the horizontal (fig. 2). 
Fig. 2. 
The camera employed was an ordinary quarter-plate camera. For our earlier 
observations the ordinary camera lens was exchanged for a single quartz spectacle 
lens with the object of avoiding the absorption of useful actinic rays by glass. This, 
however, gave imperfect definition in parts of the field away from the centre, and the 
position of the best actinic focus was also troublesome to find. And we soon found 
that the illumination was amply sufficient for the ordinary lens. This had a focal 
length of 15 centims. and was used with fidl aperture of 2'22 centims., and placed at 
such a distance from the splash as to give an image three-fourths (linear) of the real 
size. 
The plates used were Thomas’s cyclist, and were developed in complete darkness 
by treatment with a saturated solution of eikonogen for about 40 minutes, according 
to the advice kindly given us by Professor Boys. 
In order to identify the photographs it was necessary to number the negatives, 
and, to escape the difficulty of doing this in pitch darkness by scratching the film or 
