HarrLtey—On some Devices facilitating the Study of Spectra. 241 
simple Mecke burner shows the essential features of the calcium 
spectrum. The slit was sufficiently narrow to divide the two 
sodium lines when very minute quantities of sodium were 
present. 
A Device for showing Chloride Spectra.—When an oxide is 
supported in the flame of a Mecke burner, it may be made to 
yield a chioride spectrum by introducing a few fibres of asbestos 
or tobacco-pipe upon which is erystaliised some ammonium 
chloride. ‘The effect is, however, evanescent; and to operate con- 
tinously over long periods, the burner is supplied with gas mixed 
with the vapour of chloroform in exactly those proportions which 
give the best effect. ‘The gas may be taken from two separate 
taps, or from a tube with a by-pass; one-half of the gas to be 
burnt goes through a bottle containing sponge saturated with 
chloroform. The outlet tube from the bottle is joined to one 
end of a , piece; the gas is joined to the other; while a single 
tube goes to the burner. By regulating the two taps, the most 
brilliant spectra may be made to continue for several hours 
without trouble ; and the spectra may be photographed. 
On measuring Spectra—In making observations of the visible 
spectrum, measurements made with cross-wires in the eye-piece of 
the telescope are seldom quite concordant when series of measure- 
ments are made throughout the whole spectrum, first in one 
direction and then in the other; the differences are greater in the 
measurements of bands than in those of lines. This is due to two 
causes, the one an alteration in the focus of the eye; the second, 
slight variations in the width and intensity of the bands. 
To counteract the first difficulty I have had two instruments 
made with graduated draw-tubes, and have marked the focus as 
determined for red, yellow, green, blue, and violet lines, such as 
lines of potassium, lithium, sodium, thallium, strontium, calcium, 
and a spark-line of magnesium. Of course the focusing is adapted 
to only one eye-piece. In measuring green rays the telescope is 
adjusted for the thallium-line as marked upon the scale ; and other 
measurements are easily made on either side of this. Hach 
observer must focus for himself. In the measurements of bands 
the Mecke burner offers a decided advantage over the ordinary 
Bunsen flame, because it is not subject to fluctuations in tempera- 
ture, and is on the whole hotter, being about 1400° C. throughout 
