142 
ME. W. HUGGINS ON THE SPECTEA 
construction, sometimes two, at others four cells having been employed. Each of these 
cells has 33 square inches of acting surface of platinum. With two such cells the 
induction spark is 3 inches in length. A condenser is connected with the primary 
circuit, and in the secondary a battery of Leyden jars is introduced. Nine Leyden jars, 
each surface of each of which exposes 140 square inches of metallic coating, were 
employed. These are arranged in three batteries of three jars each, and the batteries 
are connected in polar series. 
The metals were held in the usual way with forceps. The nearness of the electrodes 
to each other, their distance from the slit, and the breadth of the latter were varied to 
obtain in each case the greatest distinctness. The amount of separation of the elec- 
trodes was always such that the metallic lines under observation extended across the 
spectrum. The two sets of discharging-points were arranged in the circuit in series. 
6. Some delay was occasioned by the want of accordance of the earlier measures, 
though the apparatus had remained in one place and could have suffered no derange- 
ment. These differences are supposed to arise from the effect of changes of tempera- 
ture upon the prisms and other parts of the apparatus. This source of error could not 
be met by a correction applied to the zero-point of measurement, as the discordances 
observed corresponded, for the most part, to an irregular shortening and elongation of 
the whole spectrum. 
The principal air lines were measured at one time of observing, during which there 
was satisfactory evidence that the values of the measures had not sensibly altered ; and 
these numbers have been preserved as the fiducial points of the scale of measures. The 
lines of the spectra of the metals have been referred to the nearest standard air line, so 
that only this comparatively small interval has been liable to be affected by differences 
of temperature. Upon these intervals the effect of such changes of temperature as 
the apparatus is liable to be subjected to is not, I believe, of sensible amount with 
the scale of measurement adopted. Ordinarily, for the brighter portion of the spectrum, 
the width of the slit seldom exceeded inch ; when this width had to be increased 
in consequence of the feebler illumination towards the ends of the spectrum, the measure 
of the nearest air line as seen in the compound spectrum was again taken, and the places 
of the lines of the metal under observation were reckoned relatively to this known line. 
By this method of frequent reference to the principal air lines the measures are not 
sensibly affected by the errors which might have been introduced from the shifting of 
the lines in absolute position in consequence of alterations either in the width of the 
slit, in the place and direction of the discharge before the slit, or in the apparatus from 
variations of temperature, flexure or other causes. 
The usual place of the electrodes was about ’7 inch from the slit, though occasionally 
they were brought nearer to the slit. When they are placed in such close proximity, 
the sparks charge the spectroscope by induction, but the inconvenience of sparks striking 
from the eyepiece to the observer may be prevented by placing the hand upon the 
apparatus, or putting, the latter into metallic communication with the earth. 
