238 Photographs of the Spark Spectra of Elementary Substances 
DescRIPTION OF PLATES XVI., XVII, AND XVIII. 
The plates, which are as nearly as possible facsimile reproductions of the original photographs on 
glass, have been executed by the Woodbury Type Printing Company. 
Bands and lines common to all spectra are due to air. Sixty-six have been counted. They are 
chiefly seen at the least refrangible end of the spectrum in the blue and violet region. 
PLATE XVI. 
(1.) Spectrum of Magnesium. The least refrangible ray which is visible in the violet consists of two 
nebulous but intense dots. In Watts’ “Index of Spectra” the wave-length is given as 448-1 (Thalen) 
and. between 448°] and 447-9 (Kirchoff). Next occurs a triplet, which has the appearance of one 
nebulous line unless minutely examined. M. Cornu designates the four triplets in the magnesium 
spectrum, b in the green, b’ between lines K and L, b” between P and Q, and b’” between S and T of the 
ultra-violet solar spectrum. ‘This is b’ with wave-lengths 383-74, 383-13, and 382°8. The next group 
of strong lines consists of a pair, with wave-lengths 293-49 and 292-67, a single line 285°3, and a group 
of four lines 280°13, 279-71, 279-45, and 278:99. For M. Cornu’s measurements see Annales de |’ Ecole 
Normale, Vol. 9, 1880; also “ Determinations des longueurs d’onde,” &e.; Archives des Sciences 
Physiques et Naturelles de Genéve (3), I., pp. 119-126. Lastly, a group of five fine lines, continuous, 
equidistant, and very distinctly seen in the glass originals ; three of them appear in the prints, namely 
—Ist, 278-22; 3rd, 277-95 ; 5th, 277-69—Liveing and Dewar, “ Investigations on the Spectrum of 
Magnesium.”—Proc., Roy. Soc., No. 213, 1881. 
(2.) Zinc. The most refrangible rays in the spectrum of this metal were measured by M. Cornu, but 
they do not appear in the prints of these photographs, being too feeble ; their feeble character bemg due 
to the absorptive power of the six feet ofair through which the rays passed. 
(3.) Cadmium. The lines of this metal, which were first measured by M. Mascart, have since bcen 
re-measured by M. Cornu. ‘The basis of M. Mascart’s measurements was the number of Fraunhofer for 
the line D—namely, 588°8. M. Cornu took measurements from the most refrangible of the D lines with 
the wave-length 588°89. M. Cornu’s numbers are the following :—Wo. 9, 360°9 ; No. 10, 346°68 ; Wo. 
11, 340-15; Wo. 12, 324:7; Mo. 17, 274-77; No. 18, 257-23 ; No. 22, 23218; Wo. 23, 231:35 ; Wo. 
24, 226-55 ; No. 25, 219-45 ; Wo. 26, 214-41. This last is not visible in these photographs, the other 
two adjacent lines have wave-lengths of 324°8 and 325°8. 
(4.) Aluminium. The two continuous and extended lines near the strong air line Vo. 8 in the spectrum 
of Cadmium 398-56 (Mascart) are figured in M. Cornu’s map. Their wave-lengths are 396-06 and 
394-33. Two other lines similar in character, but more refrangible, appear to be identical with a pair 
with wave-lengths 309-15 and 308-5. These lines are about in the same position as a faint triplet b’ in 
the Magnesium spectrum, the least refrangible ray of which has a wave-length of 309-6. (Cornu). 
(5.) Indium. The two least refrangible lines in the blue and violet have been measured by Thalen, 
Wave-lengths 450-9 and 410-1. 
(6.) Thallium. (7.) Copper. (8.) Silver. (9.) Mercury. 
PLATE. XVII. 
(10.) Graphite. The spectrum of Graphite contains at most only twelve of the shortest possible lines or 
“dots.” Eleven of these are visible in the prints when examined with a powerful lens. 
(11.)° Zan. 
(12.) Lead. The least refrangible line is visible, and has been measured by Kirchoff; wave-length 
438°7 to 438-6. 
(13.) Tellurium. (14.) Arsenic. (15.) Antimony. (16.) Bismuth. 
PLATE XVIII. 
(18.) Zron. The spectrum of this element contains over 600 lines visible in the original photographs, 
exclusive of air lines. The total number of lines belonging to the metal, and capable of production with 
aone prism spectroscope, cannot, however, be fewer than 1,000, since, by the use of more sensitive 
plates, spectra increased by one-half in length may be photographed, the new region being crowded with 
lines. 
(19.) Mickel, 225 lines. (20.) Cobalt, 490 lines. 
(21.) Palladium, 333 lines. (22.) Gold. 
(23.) A repetition of the Alwminiwm spectrum. 
