THE ULTRA-VIOLET SPECTRA OF THE ELEMENTS. 
191 
know the distance of the plate from the centre of the object-glass of the telescope. 
It was found by measurement that the distance from the object-glass to the front of 
the photographic plate, when the sliding tube was at the 100th division, was 439 
millims., and the thickness of the lens at its middle 2*5 millims., which, divided by 
the refractive index of quartz for the high rays observed is very nearly 1 - 5 mi Him., 
making the distance of the photographic plate from the optical centre of the object- 
glass very nearly 440'5 millims. From this the formula tan 1 II. III. IV. * * VII. * IX. 
I) 
3068‘5 +4a 
was deduced 
for the angular distance of a line from the axis of the telescope when the difference 
of the micrometer readings of the two images of the line is D, and the number of 
divisions of the scale of sliding tube at which the telescope was focussed was a. To 
determine whether the angle so found was to be added or subtracted from the reading 
of the circle, all that was necessary was to observe whether the micrometer reading of 
the upper or lower image as seen in the microscope were the greater. The angular 
position of the line having thus been found on each side of the collimator, the wave¬ 
length was deduced by the ordinary formula. 
The following tables give the measurements made, the calculated angles, and the 
wave-lengths deduced. 
The quantities recorded in the several columns is as follows :— 
1. The mark of the particular photographic plate. 
II. The reading of the circle, giving the position of the telescope. 
III. The reading of the scale of focussing tube. 
IV. The measured distance of the two images of the line in hundredths of an inch. 
Y. One half this distance reduced to angular measure with the sign + or — 
according as it is to be added or subtracted from the reading of the circle to give the 
angular position of the line. 
YI. The angular position of the line. 
VII. The mean values of the angular positions so found. When several sets of 
plates have been taken at different times, or with varied adjustments, the means for 
the several sets are given separately. 
YIII. The deviation from the normal to the grating of the line as deduced. 
IX. The wave-length deduced or adopted. When different sets of plates give 
different values, these different values are inserted in brackets. 
The figures belonging to different lines are separated by horizontal spaces. 
In the case of plates marked with a figure less than 172 the arc was employed to 
give the light, in the case of all plates with the mark 172 and upwards the spark was 
employed. 
