106 PROCEEDINGS OF THE ACADEMY OF [Feb., 



MICRO-SPECTROSCOPIC OBSERVATIONS. 



BY F. J. KEELEY. 



Since Sorby devised the micro-spectroscope, nearly fifty years ago, 

 an immense amount of work has been done with this instrument, but 

 principally in the study of organic coloring matters in solution or 

 section, which could be examined by transmitted light. It was 

 understood that the micro-spectroscope could likewise be employed 

 in examining objects by reflected light, but this phase of its utility 

 has been almost completely neglected. With transmitted light, it 

 produces results from very minute quantities of material, but other- 

 wise offers little advantage over an ordinary spectroscope of low 

 dispersion. With reflected light, it is also applicable to the study of 

 small quantities, but has a number of additional advantages. Absorp- 

 tion bands are usually more distinct; a small crystal on a mineral 

 specimen or a gem embedded in an opaque setting may be examined 

 without disturbing or damaging it ; and phenomena such as iridescence 

 on opaque substances may be studied. 



The microscope employed in connection with this instrument should 

 always be a binocular, which permits the object to be examined through 

 one tube while the spectroscopic ocular is applied to the other. The 

 illumination should preferably be rather brighter and whiter than is 

 generally necessary for the examination of opaque objects, and is 

 best concentrated by means of a parabolic silvered reflector attached 

 to the objective. For preliminary examinations, the older form of 

 micro-spectroscope, as made by Browning or Beck, is preferable, as 

 very faint bands are more readily noted in its short bright spectrum, 

 but for further study and recording the spectra, the Zeiss model, with 

 photographed scale, is more desirable. 



There are two classes of spectra that can be advantageously studied 

 under the conditions specified, those showing absorption bands, 

 which permit the identification of certain minerals and gems; and 

 interference spectra, which assist in determining the cause of luster 

 and iridescence. 



Of the first class, the characteristic spectrum of didymium is the 

 best known and most useful. While didymium has been resolved 

 into two elements, neodymium and praseodymium, for the present 



