6 
ADLER. 
Almost from the beginning of his astronomical work he 
had devoted his attention to the sun, his investigations 
being chiefly astrophysical in character, and among his 
earlier observations in this field were his sun-spot studies, car- 
ried on about 1873. From that time on until 1880 he was 
engaged in minute telescopic study and drawing the details 
of the surface of the sun, and especially of sun-spots. Photog- 
raphy had not begun to be used for such purposes, and his 
skill and accuracy in making drawings of observations of 
these phenomena were particularly valuable. Indeed, it is 
declared by astrophysicists that his sun-spot drawings made 
at Allegheny prior to 1875 are even yet to be regarded as the 
best recorded evidence of their structure. I learn from Mr. 
Abbot, of the Astrophysical observatory, that “Professor G. 
E. Hale, who has enjoyed the choicest opportunities for ex- 
amining the sun, both with the forty-inch reflector of the 
Yerkes observatory and with the horizontal telescope on Mt. 
Wilson, and also during various expeditions to high moun- 
tain peaks, says that in the best views of sun-spots he has 
ever had, the better they were seen the more nearly have 
they appeared as shown in Langley’s drawings.” In spite of 
this great power of direct personal observation, he was quick 
to appreciate and to employ the aids which photography 
lends to this research, though it should be said that the 
standard illustration of a sun-spot which appears in most 
of the text books and works on astronomy of the present time 
is one drawn by Mr. Langley with his own hand at Alle- 
gheny in December, 1873. The following statement of his 
continued work in this field during his Allegheny period 
was prepared recently for publication in a general encyclo- 
pedia, and, having had the advantage of his own revision, 
it is taken as an authoritative statement of his researches: 
“About 1875 he began to devote much attention to the 
measurement of the heat spectra of the sun and other 
sources of radiation. Convinced after long experience with 
the thermopile of the futility of attempting to discriminate 
the effects of narrow portions of the spectrum by means of 
any heat-measuring apparatus then employed, he sought to 
