M. A. Wullner on the Absorption of Light. 47 



I lately received from MM. G. and J. Mertz in Munich a 

 prism of flint-glass of very great dispersive power. This led me 

 to resume the experiments of the previous year, since I could 

 hope by its means to determine at least the position of the pre- 

 viously observed dark lines in the iodine spectrum. 



With this view the prism was fitted in one of Steinheil's spec- 

 trum apparatus, which was provided with a scale in the ordinary 

 manner, in order to be able to fix by its divisions the light or 

 dark lines in the spectrum seen through the apparatus. 



And first the position of the absorption bands in the spectrum 

 of daylight which had passed through iodine vapour was deter- 

 mined. The bands were seen about as far as the Fraunhofer's 

 line F ; the most refrangible part of the spectrum was certainly 

 enfeebled, but bands I could not detect in it. 



So long as the iodine vapours were not too dense, the absorp- 

 tion bands appeared in my prism, between the Fraunhofer's lines 

 C to almost F, as almost equally distant black bands, so that the 

 bright intervening space was of almost the same breadth as the 

 black bands. If the iodine vapours became denser and denser, 

 the character of the absorption spectrum in the red and yellow, 

 as far as green, did not change ; the individual dark bands were 

 only darker and a little broader. At the same time the green, 

 as the iodine vapour became denser, appeared to become covered 

 with dark red, which gradually became denser, and finally com- 

 pletely extinguished the green ; but as long as the bands were 

 visible, they did not appear to have become broader than the 

 others. 



It must, however, be mentioned, in reference to the absorption 

 in the green part of the spectrum, that even from the begin- 

 ning the bright spaces between the absorption bands appeared 

 darker, so that it was difficult to fix sharply the individual dark 

 lines. 



The following small Table contains the position of a number 

 of bands determined on the scale of the apparatus — that is, of 

 those which were visible from near C to about D \ E. To obtain 

 a representation of this, it may be mentioned that the scale is 

 divided into 250 parts, of which the spectrum covered about 200. 

 The spectrum appears to lie upon the scale, whose divisions 

 are feebly illuminated; but the dark bands can be seen above, 

 between, and below the divisions, so that their position can be 

 determined with great certainty. 



view. On the basis of the theory of absorption in question, I predicted, 

 together with the second iodine spectrum, one corresponding to a lower 

 degree of heating as negative image of the absorption spectrum ; and in- 

 versely, I expected in the realization of this prediction a beautiful confirma- 

 tion of the theory of absorption." 



