180 Prof. R. Bunsen on the Phenomena observed 



the faces P, and through which the light passed, had a thick- 

 ness of 1*55 millim. as measured by a spherometer. Hence the 

 values of d and / are 



d=l«2563, i=l-55. 

 Solutions of three didymium salts were examined; one of sul- 

 phate, one of acetate, and one of chloride. In all three the values 

 of </j and / t were 



^ = 0-03414 grin., / 1 = 57'1 millim. 

 The tube (Plate II. fig. 10) used to contain these solutions consists 

 of a thick-walled glass tube of 6 millims. internal diameter, into the 

 ends of which two glass stoppers with plane parallel terminal sur- 

 faces were ground. These stoppers were cut out of a piece of plate 

 glass, and fitted by accurate grinding into the tube. In order to 

 prevent the tube from crackingwhen completely full of liquid, from 

 expansion caused by rise of temperature, a side-tube was melted 

 into the middle of the tube, and this was closed at the end and filled 

 with air. This side-tube also served as a handle to hold the tube 

 horizontal in the cork of the stand, so that the tube could be turned 

 upon the axis of the side-tube and then taken out to be cleaned 

 or refilled. Fig. 1 (Plate II.) gives the position of Fraunhofer's 

 lines, to which the three first spectra are to be reduced. Fig. 2 

 represents the spectrum obtained by polarized lamplight with the 

 small apparatus when the crystal is placed in the orthodiagonal 

 position ; and fig. 3 gives the spectrum by polarized light when 

 the crystal is placed in the klinodiagonal position. Non-polarized 

 light ought to give a spectrum the mean of these two ; but it 

 cannot be distinguished from that represented by fig. 3. The 

 differences between figs. 2 and 3 are best seen in the three chief 

 groups of bands near Fraunhofer's lines D, E, and F, as in these 

 groups the bright spaces between the well-defined sharp absorp- 

 tion-bands in fig. 3 become darker in fig. 2, and thus each group 

 of bands attains an indistinct and totally different appearance. 



In order to investigate more accurately the changes which 

 occur when polarized light is used, the single groups of bands at 

 D, E, and F were examined by sunlight in the larger instrument. 

 Fig. 5 represents the group of bands near D when the crystal 

 is examined by polarized light in orthodiagonal position ; fig. 6 

 the same group in klinodiagonal position. The spectrum fig. 6 

 differs from that of fig. 5, inasmuch as the spaces between the 

 single dark bands become brighter, and the band at 1100 dis- 

 appears, and in its place a new band appears between 1090 and 

 1095. The group of bands near E undergoes a still more re- 

 markable change. The form of spectrum seen by polarized light 

 in orthodiagonal position (fig. 12, Plate III.) changes to the 

 form represented by fig. 13 when the position of the crystal is 



