876 
PEOEESSOE W. A. MILLEE ON THE PHOTOGEAPHIC 
but the more refrangible portion was deficient in power, and no rays were reflected 
beyond 159-5 (63 divisions), the point at which the other metals also failed. I therefore 
abandoned the attempt to substitute a speculum for the lens, with which latter I 
succeeded subsequently in obtaining a field sufficiently flat for the purpose. 
24. The reflexion from the surface of transparent objects was so scanty that, of course, 
no idea was entertained of using such bodies as mirrors ; but it may be worthy of notice 
that a feeble spectrum was obtained from surfaces of quartz, windoiv-qlass, and Iceland 
spar extending to 159-5, or over a length of 63 divisions of the scale — that is to say, 
fully as far as the majority of the metals. The quantity of the reflected rays was small, 
but its quality was similar to that of the rays reflected from metallic surfaces. 
§ 3. PHOTOGEAPHIC EEEECTS OF THE ELECTEIC SPECTEA OF DIFFEEENT METALS 
TAKEN IN AIE. 
a. Pure Metals. 
25. I have spent a considerable time in endeavouring to procure exact photographs of 
these spectra, inasmuch as the spectrum of a metal is a constant not less important than 
its density or its fusing-point ; and it frequently furnishes the means of identifying an 
element under circumstances in which no other method at present known is practicable. 
Kirchhoff, in his elaborate and masterly researches on the constitution of the solar 
spectrum, has, as is well known, published in minute detail a map including the lines of 
a large number of the metals. He has, through a limited portion of the visible spectriun, 
laid down the position of the bright lines of certain metals coincident with particular 
dark lines of Fraunhofer, with a precision best appreciated by those who have followed 
him with most minuteness. 
Much yet, however, remains to be done even for the rays which fall -within the range 
of the -visible spectrum ; and for those which are beyond the limits of ordinary vision, the 
whole yet remains to be examined. 
The lines of each spectrum are so numerous and so close together, that it would be 
impossible without a sacrifice of time, that would scarcely be justifiable, to obtain accu- 
rate impressions of them by eye-drawing. Indeed, except by the process of photography, 
these lines can only be rendered visible by the aid of a fluorescent screen, under which 
circumstances the minute details are almost necessarily lost even by the most careful 
observer. 
The photographs of these spectra were obtained by an arrangement of the quartz 
prism and lens, identical with that already described (6), wires, plates, or irregular 
fragments of the metal, according to circumstances, being supported in brass forceps 
connected with the secondary wires of the induction-coil. The interval traversed by the 
spark was in each case about a quarter of an inch, and the slit was placed at a distance 
of half an inch from the line traversed by the spark. 
The specimens of gold, silver, mercury, copper, bismuth, antimony, zinc, tellurium, 
thallium, and lithium employed are believed to have been pure. The timgsten. 
