in the Absorption- spectrum of Didymium. 181 



altered to the klinodi agonal one. The five weaker bands disap- 

 pear almost entirely, whilst only a slight alteration is noticed in 

 the shade of the bands between 1557 and 1570; but at 1476 a 

 new band appears. The alterations produced in the group of 

 bands near the line F are the least visible. Fig. 17 represents 

 this group : it exhibits such slight alterations in the relative 

 brightness of the absorption-bands viewed with polarized light, 

 either in the orthodiagonal or klinodiagonal position of the 

 crystal, that these differences cannot be distinctly made out, 

 especially when viewed by the somewhat weak light which the 

 solar spectrum possesses near F. The same observation was 

 made with several other bands ; and their description, for a similar 

 reason, is not given. 



All these differences in the absorption-spectra occurring with 

 polarized light resemble the absorption- phenomena which tour- 

 maline exhibits under similar circumstances. The property 

 which crystallized didymium-sulphate possesses, of absorbing 

 differently the ordinary and extraordinary rays, is in its nature 

 identical with that which makes tourmaline so valuable in optical 

 experiments. 



The solution of the crystals in water gives another spectrum, 

 differing, again, from both the foregoing. This is represented on 

 Plate II, fig. 4. Here also the three chief groups of bands are 

 situated near D, E, and F, but they are still more confused than 

 in the spectrum obtained when the crystal is placed in the ortho- 

 diagonal position (fig. 2). 



Some of the maxima of absorption do indeed show a small but 

 perceptible change of form : thus, for instance, the band 27 to 

 31 of the crystal-spectrum disappears, whilst a new but faint 

 band becomes visible at 30 in the spectrum of the solution. 



Very remarkable and noteworthy are the small alterations in 

 position which occur in the minima of brightness in the didy- 

 mium-spectrum, dependent upon the nature of the compound in 

 which the metal occurs. These changes are too minute to be 

 seen with the small, though seen with the larger instrument; 

 I have as yet only investigated them completely in the case of 

 three didymium-salts, viz. the chloride, sulphate, and acetate. 

 It is, however, more than probable that these same phenomena 

 will also be found to occur with other solutions, and with the 

 absorption- spectra of other crystals of didymium-salts, and per- 

 haps may be exhibited with the luminous spectra of the oxide 

 and other compounds of didymium. The difficulty of obtaining 

 a variety of well-crystallized didymium-salts of sufficient trans- 

 parency has prevented me from pursuing the subject further in 

 this direction. Plate II. figs. 7, 8, and 9 represent the group of 

 bands near D of didymium chloride, sulphate, and acetate, in the 



