ON THE MEASUREMENTS OF WAVE-LENGTHS. 
65 
being on the same stand below it, the arm by which they were held being capable of 
such motion that they could be renewed or replaced without disturbance of the fixed 
cadmium point. The image of the spark was projected on to the slit by a lens of 
3 inches focus, which was also immoveably fixed. All lines photographed could be 
measured with reference to those in the spectrum of cadmium. The slit of the 
collimator, which was not more than joVoth of an inch in width, was protected from 
dust by being covered with a thin plate of quartz. Photographs were taken of the 
first order, both to the right and to the left of the plain reflection of the slit. The 
average period of exposure was an hour to an hour and a half. 
The spectrum of one order overlaps that of another, but this is of no consequence, 
because the lenses being uncorrected for chromatic aberration, and the spectra of 
different orders having different foci, only one image is visible on the developed plate. 
The developer used was made with pyrogallic acid and potassium bromide. 
Method of measuring the spectra .—When a series of photographs had been obtained, 
the distances between the lines of the various spectra were accurately measured by 
means of a microscope and a dividing engine with a screw 30 inches in length. The 
arrangement by which a forward motion only is given to the screw was thrown out of 
gear, and a divided wheel, 4 inches in diameter, with a handle attached, was placed 
at the opposite end of the screw. Each division on the wdieel as it passed the pointer 
registered a longitudinal motion of the stage equal to 5 -^ 0 -th of an inch, and it was 
easy to read to Tcumofh- The measurements were certainly accurate to ^oiroth of an 
inch even when working on lines of different intensities. The microscope, which was 
placed on the stand of the machine, had a magnifying power of 25 diameters. Less 
than this is insufficient, and more is unsatisfactory, except in special cases. 
A plate-glass stage was fitted to the carrier of the dividing engine, and by means 
of screw clamps the photographs were secured to this. The photographs must be so 
adjusted that a line passing from end to end of the spectrum and dividing it into two 
parts longitudinally lies parallel to the axis of the screw, otherwise the lines will not 
all occupy the same position with respect to the cross lines in the field of the 
microscope. 
Again, it is necessary that the photographs be taken on patent plate-glass so as to 
present a perfectly flat surface, and the plates are more suitable if selected with regard 
to equality of thickness at each end. Such curvature as is ordinarily to be seen in 
flattened crown-glass would yield inaccurate measurements, and if the plates be not 
of the same thickness throughout, the two ends of the spectrum, when the plate was 
in position on the glass stage, would not be in the same horizontal plane, and so by 
the motion of the screw the lines of the spectrum would soon travel out of focus. 
Freedom from spherical aberration, which facilitates the measurement of the lines, is 
secured by using lenses of unusually long focus. 
Three plates were taken for each spectrum, the first included all rays lying between 
the cadmium lines 6 and 12 (\=4676'7 and 3249’5), the second included Cd 11 and 
MDCCCLXXXIV. K 
