SPECTRA OF VARIOUS SUBSTANCES. 



253 



that this is by no means the case, and a reference to 

 Fig. 219 will show this clearly. 



In the illustration, A represents the absorption spectrum 

 of oxalate of didymium viewed by reflected light. The 

 oxalate is a white powder, yet the six bands are very 

 plainly defined even with ordinary instruments, while with 

 a good set of prisms the broad band at the red end of the 

 spectrum may be resolved into several dark lines. 



RED 



.VIOLET 



FIG. 219. 



The absorption bands of didymium were first described 

 in 1858 by Dr. J. H. Gladstone, and are remarkable for 

 their extreme sharpness. 



At B is shown the spectrum of the deep blue solution 



