Se 
a 
W. LeConte Stevens—fecent Progress in Optics. 381 
even better, perhaps, than Langley’s previous work on the sun. 
But more recently with the collaboration of several able 
assistants, and more particularly the great ingenuity and 
mechanical skill of Wadsworth, the sensitiveness of Langley’s 
galvanometer has been so exalted, and the bolometer connected 
In such manner with photographic apparatus, as to make it an 
automatically controlled system, by which an hour’s work now 
brings results superior in both quantity and quality to what 
formerly required many weeks or even months.* Not only isan 
entire solar energy curve now easily obtained in a single day, but 
even a succession of them. It becomes thus possible by com- 
parison to eliminate the effect of temporary disturbing condi- 
tions, and to combine results in such a way as to represent the 
infra-red cold bands almost as accurately as the absorption 
lines of the visible spectrum are indicated by use of the 
diffraction grating. It will undoubtedly become possible to 
determine in large measure to what extent these bands are due 
to atmospheric absorption and which of them are produced by 
absorption outside of the earth’s atmosphere. 
With the diffraction grating, supplemented by the radio- 
micrometer, Percival Lewist has recently investigated the infra- 
red spectra of sodium, lithium, thallium, strontium, calcium 
and silver, attaining results which accord well with the best 
previously obtained by those who had employed the bolometer, 
and which demonstrate the exceeding delicacy of the radio- 
micrometer as an instrument of research. 
The Visible Spectrum. 
To follow out all the applications of the spectroscope that 
have resulted in recent additions to our knowledge would 
carry us far beyond the scope of a single paper. It is possible 
only to make brief mention of a few. 
For a number of years Rowlandt has been investigating the 
spectra of all the chemical elements, photographing them in 
connection with the normal solar spectrum, and reducing them 
to his table of standards, which is now accepted everywhere. 
The work is of such magnitude that years more must elapse 
before its completion. It now includes all wave lengths from 
3722 to 7200, and of these the list already published extends 
as far as wave length 5150; or, from ultra violet nearly to the 
middle of the green. ; 
Through the spectroscope chiefly has been established during 
the present year the discovery of the new atmospheric element, 
* Langley “‘On Recent Researches in the Infra-red Spectrum,” Report of 
Oxford meeting of British Association, 1894. 
+ Astrophysical Journal, June, 1895, p. 1, and Aug., 1896, p. 106. 
t Ibid., Jan. to Aug., 1895. 
