1911.] NATURAL SCIENCES OF PHILADELPHIA. 107 



purpose the absorption bands which have become classical as those of 

 didymium can be regarded as indicating the presence of the whole 

 group, including cerium, lanthanum, erbium and terbium, the last 

 two of which likewise have characteristic spectra of their own, not 

 usually visible in minerals to be considered. 



Under most favorable conditions, the spectrum of "didymium" 

 contains nine absorption bands, centering at about the following wave 

 lengths, .675, .622, .591, .575, .532, .521, .508, .480, .471 micron, the 

 first two of which, in the red, are not usually visible in a solution or 

 section examined by transmitted light, but quite distinct when a 

 crystallized salt or a mineral such as lanthanite is viewed by reflected 

 light. Position of bands is given as determined by the photographed 

 wave-length scale on Zeiss micro-spectroscope, which was itself ad- 

 justed on the D line from a sodium flame, and is probably sufficiently 

 accurate for the purpose. For reference, all spectra should be recorded 

 diagrammatically on paper ruled with lines corresponding to the scale, 

 showing width of band, whether edges are sharp or misty and whether 

 the darkest part of band is central or eccentric. 



In determining the presence of this element, it is of course unneces- 

 sary that all the bands be visible. If two or three, including the 

 strongest at junction of yellow and green, can be seen and their pos- 

 tion accurately measured, it is sufficient. 



Many of the minerals characterized by the "didymium" spectrum 

 occur in microscopic crystals, sometimes not sufficiently well developed 

 to determine with certainty, but if the presence of these absorption 

 bands demonstrates that the mineral is one of the comparatively few 

 which contain the cerium group, there will always be sufficient indica- 

 tions of its identity from color, luster or such few crystal faces as may 

 be present. In this manner, I have been able to confirm the discovery 

 of monazite in Delaware County, where but a few small crystals have 

 been found, and likewise to identify microscopic crystals of parisite 

 from a new locality in New England. 



Another element which shows a characteristic spectrum in most of 

 its compounds is uranium, the several equidistant absorption bands 

 in the blue end of spectrum, assisting in determining poorly crystal- 

 lized microscopic occurrences of such minerals as autunite and tor- 

 bernite. 



A mineral that has afforded material for extensive study of spectra 

 is zircon, which has long been known to show absorption bands, the 

 cause of which seems unexplained. These bands center at about 

 wave lengths .687, .650, .615, .586, .561, .538, .508, .478 and .455 



