232 Photographs of the Spark Spectra of Elementary Substances, 
and their insignificent character when of lesser wave-length than 380°0. The 
graphic characteristics of the lines seen in the spectra of various substances may 
be described thus, they consist of strong and faint lines, which again may be 
classified as, (1), Continuous lines; (2), Discontinuous; (3), Extended lines; (4), 
Blotted lines, or lines with a nimbus; (5), Nebulous lines and bands. 
(1.) Continuous lines are those most abundant in the less refrangible portions 
of the spectra of iron, cobalt and nickel; they extend the whole length of the 
spark and are accurate representations of the slit. 
(2.) Discontinuous lines are those whose length is not so great as the distance 
between the two electrodes. They may proceed either from the positive or the 
negative electrode, or from both, and may be called long and short lines, as in the 
case of certain lines of cadmium ; or ‘they may consist simply of dots, as in the 
case of the more refrangible rays in the spectrum of zine. 
(3.) Extended lines are those like the 17th line of cadmium, which being 
sharp lines extend above and below the edge of the spectrum, or in other words 
above and below the points of the electrodes. The extension of the line is caused 
by the glow or “glory ” surrounding the electrodes being very intense. These lines 
are the longest and are continuous. 
(4.) Blotted lines, or those with a nimbus, are well represented by the least 
refrangible ray in the photographed spectrum of magnesium, the wave-length of 
which lies between 448°1 and 447°9. It is visible in the violet region. The 
nimbus is characteristic of a great intensity of brilliancy or of chemical activity. 
The nimbus of a line may be much lessened in width, and in fact made to assume 
the character of nothing more than a slight widening of the line at each end, by 
greatly reducing the width of the slit of the spectroscope, as for instance, from the 
soo to the 100 of an inch. 
(5.) Nebulous lines are those which are destitute of the sharp clear-cut 
appearance of most metallic lines. The air bands and lines at the less refrangible 
end of all the photographs are for the most part nebulous in character. They 
vary considerably in intensity. Irrespective of the air limes constantly present, 
nearly all varieties of lines may be noticed in one spectrum, as for instance, in that 
of cadmium. 
It is necessary to state the conditions under which these spectra were obtained, 
and then to describe carefully the lines of cadmium, so that when an apparatus has 
been adjusted so as to yield photographs exhibiting the characters of these lines 
distinctly, it will be known that it is accurately focussed for all spectra taken in a 
similar manner. 
Manipulation necessary to secure-sparks of constant intensity, dc., &e.—-The 
battery power, size of coil, and condensing surface, must all be proportional to the 
distance between the electrodes. When the battery power is too weak, the stream 
of sparks is intermittent, and with badly conducting electrodes may cease- 
