22 
DES. J. PLtJCKEE AND J. W. HITTOEE ON THE 
towards the less as towards the more refracted part of the spectrum. Ha remains 
almost unchanged after H y has passed into an undetermined large violet band, and H/3 
extended its decreasing light on its two sides. On employing the Leyden jar, and 
giving to the gas in our new tubes a tension of about 60 millims., the spectrum is 
already transformed into a continuous one, with a red line at one of its extremities. 
At a tension of 360 millims. the continuous spectrum is highly increased in intensity, 
while the red line Ha, expanded into a band, scarcely rises from it. If the electric 
spark passes through hydrogen at the ordinary tension, the ignited gas on its way 
always gives the spectrum of the three expanded lines*. 
60. Even in the old spectral tubes enclosing highly rarefied hydrogen, the ground, 
from which the three characteristic lines rise, did not appear always of the same dark- 
ness ; in some instances new bright lines appeared, especially in the neighbourhood of 
the sodium-line. In resuming the subject, we pointed out the existence of a new 
hydrogen-spectrum , corresponding to a lower temperature, but having no resemblance 
at all to the spectra of the first order of nitrogen, sulphur, &c. In this spectrum, of a 
peculiar character, if fully developed, we observe a great number of well-defined bright 
lines, almost too numerous to count and represent by an engraving, but brilliant enough to 
be examined at a magnifying power of 72, after the light has passed through four prisms. 
* After Fraunhofer, and especially Dr. Wheatstone, directed the attention of philosophers to the electric 
spectrum, Masson indicated the red hydrogen-line, hut without referring in an explicit way to its origin. 
Angstrom first separated the spectrum of gas from the spectra of metal. In the diagram he gave of the 
hydrogen-spectrum, he represented, by means of curves, the intensity of light along the whole length of the 
spectrum, especially the maxima of intensity within the red, the green, and the violet. These maxima corre- 
spond to Ha, H/3, H y, here expanded into bands, the breadth of which, as well as their decreasing intensity 
towards both ends, are indicated by the extension and steepness of the curves. After one of us published 
his first researches on the spectra of ignited gases, M. van der Willigen, in operating with strong induced 
currents, determined in a similar way the maxima of intensity of the hydrogen-spectrum. 
The spectra thus obtained are not calculated to prove the connexion existing between the bright lines of 
ignited gases or vapours and Fraunhofer's dark lines of the solar spectrum. Starting, in his first communica- 
tion made to the Eoyal Swedish Academy, 1853, from the theoretical conception “ that the dark lines of the 
solar spectrum are to be regarded as an inversion of the bright lines of the electric spectrum,” M. Angstrom 
concluded the coincidence of Ha with Eraunhoeer’s line C ; but the diagram shows that this conclusion was 
not based on exact measurement. One of us, in his publication of 1859, not being guided by any theoretical 
view on this point, first announced the coincidence of H/3 with Fraunhofer's E, and fixed the position of Hy 
near G, of Ha at a distance of two minutes from C. When at a later period he made use of Steinheil’s large 
spectral apparatus, he pointed out at first sight the exact coincidence of Ha with C, Hy with a marked black 
line at some distance from G, towards E. In operating with spectral tubes, M. Angstrom confirmed these 
results. (The spectroscope employed in 1859 being a small and imperfect one, there was given to the slit an 
aperture of more than three minutes. The adjustment was made with regard to H/3. Hence the error finally 
made in determining the position of Ha may be fully explained, by the circumstance that the illuminated 
border of the slit was observed instead of the illuminated aperture itself.) — Angstrom : “ Optische Hnter- 
, suchungen,” Poggendorfe’s c Annalen,’ vol. xciv. ; “ Ueber die ERAUNHOEER’schen Linien im Sonnenspectrum,” 
Ibid. vol. cxvii. Yan der Willigen : “ Over het electrische Spectrum, Yerhandelingen der K. Hollandsehe 
Academie (Natuurkunde vii. & viii.). Plucxer, Poggendorfe’s ‘ Annalen,’ vol. evii. p. 544. 
