8 
ADLER. 
“By 1885 the solar spectrum had been followed by him to 
wave-lengths ten times as great as those ot the visib e 
spectrum, and radiations from terrestrial sources even iur- 
ther, thus overthrowing the ideas previously held ot a 
natural limit to the infra-red wave-lengths at about 1 /*. 
His extended bolometric researches on the heat spectrum of 
the moon led him to fix the maximum lunar temperature at 
little above 0°C. In his researches on these long wave- 
length spectra, Mr. Langley developed the optical possibili- 
ties and determined the constants of rock-salt, a substance 
already employed by Melloni, but whose range of usefulness 
was now very greatly extended.” 
But he did not confine himself during this time either to 
his labors in the observatory or to making their results known 
to scientific men through contributions to societies and jour- 
nals. lie had a decided opinion of the right of the world 
to know what scientific men were doing and a remarkable 
gift of presenting such knowledge to the man of average in- 
telligence. He occasionally delivered lectures in the city of 
Pittsburg, which were reported for one or another of the 
Pittsburg papers, and wrote letters to the Pittsburg Gazette 
when any unusual astronomical phenomenon which might 
bo of public interest presented itself. By 1875 his reputation 
had grown to such an extent that he was invited to lecture 
at Stevens Institute, and his papers, which had heretofore 
been published only in American journals, commenced to 
appear abroad in English and Italian periodicals and in the 
Transactions of the Academy of Sciences of the Institute of 
France; this, be it noted, within five years from the date of 
his first publication. 
llie trend of his mind toward the popularization of science 
may be judged from a paper which appeared in the Popular 
Science Monthly in 1877, entitled “The First Popular Scien- 
tific r l realise,” in which he declared that “science is not for 
the professional student only, but that every one will take 
an interest in its results if they are only put before the 
world in the right way.” The treatise was Fontenelle’s 
“Conversations on the Plurality of Worlds,” and the article, 
while holding strictly to its subject, showed something of 
