564 
MR. J. NORMAN LOCKYER ON THE 
Mawson and Swan’s “ Stellar ” plates. 
Edwards’ isochroraatic plates. 
Ilford isochromatic plates. 
Ilford special rapid plates. 
These were tested on the spot and all were found very satisfactory. Hence, some 
of each were employed for the work during the eclipse. 
The plates were all placed in the dark slides and the films carefully numbered with 
pencil on the evening preceding the eclipse. 
Since the isochromatic plates appear to answer as well for the blue and violet as 
the ordinary ones, while giving also information as to the green and yellow, it may 
be desirable to employ them exclusively in future. 
Exposure of the Plates. 
As the instrument was not provided with an exposing shutter, the exposures were 
made by covering and uncovering the prism with a piece of thick card. In this part 
of the work I was assisted by Lieutenant Shipton, R.N., and Chief Artificer Milligan, 
of H.M.S. Alecto, and the following plan was adopted after numerous rehearsals : The 
dark slide being placed in proper order on a packing case close to the camera, one 
was inserted and the figures indicating the time of the exposure were noted, the 
prism being meanwhile covered ; my calling out of the time of exposure was the 
signal for Mr. Milligan to remove the card and for Lieutenant Shipton to 
commence counting the number of seconds announced, at the termination of which he 
nave the signal “ over,” the urism was covered and the dark slide moved on for the 
next plate. In the case of the “instantaneous” exposures, “snap” was called, the 
card removed for a moment by Mr. Milligan, and the time noted by Lieutenant 
Shipton. After rehearsals the arrangements worked without a hitch. 
A pendulum clock had been provided for recording the times, but for some reason, 
probably from the sand getting among the gearing, it ceased to work satisfactorily on 
the day of the eclipse, and a navigator’s deck-watch was used in its place. 
As very little idea could be formed of the length of exposure required to give the 
best results with this instrument, the exposures were arranged by Professor Lockyer 
in a series which was repeated three times during totality. Thus, in case one par¬ 
ticular exposure was found better than the others a good record at different stages of 
the eclipse would be secured. As will be seen by reference to the table on page 566, 
the actual exposures in the series were instantaneous, 5 seconds, 25 seconds, and 
10 seconds. 
Near the middle of totality a specially long exposure of 40 seconds was inter¬ 
polated with the object of photographing the coronal spectrum free from admixture 
with the spectrum of the chromosphere. 
