S. P. Langley—Atmospheric Absorption. 175. 
photographs, of which I will only observe, that when they 
were taken, the air-mass at noon was 1°09 and in the afternoon, 
1°60, so that all this increase of telluric lines, came with a very 
little tes a of the absorbing air, and is but a small part of 
at we sbould see nearer sunset. Evidently the noon spec- 
ur oat be less bright, not only for the telluric lines dis- 
tinctly seen, but for those indistinctly seen individually, or latent 
on which come out as separate individual lines when 
the sun is lower. It results from what has just been onid, then, 
hundreds of these alternations (and necessarily too small), 
and yet more than this, that the smallest part included in the 
field of the experiment, whether the telluric lines are sepa- 
rately visible, or whether they are only latent there, is filled 
with alternations of transmission and absorption, and therefore, 
according to our previous demonstration, the mean result, even 
when obtained by a linear esa or bolometer, anya still 
indicate too feeble an eed te I speak now only of the 
Zero. e previous criticism applies then, though in a less 
tig to the rt investigations where two or three coefficients 
een , and even to investigations with the linear 
the linear bolometer ; but even ‘this strip, when laid down ina 
considerably dispersed spectrum, covers more than the distance 
between the D lines; and if we fix our attention on that well - 
nown region as a type, we see that this hair-like line itself 
covers in this narrow interval alone, at least a dozen alternations 
between brightness and almost total extinction, so that though 
in respect to wave-lengths we may be said to measure approxi- 
to this local absorption, we do not. I am convinced -that we 
do not know what this absorption really is in amount, but T 
