516 Prof. S. P. Langley on the Invisible 



in most of the visible spectrum by fine lines, which begin 

 already to show aggregation into a broader absorption-band 

 on the borders of the infra-red (e. g. the well-known " A " of 

 Fraunhofer), is, as we pass further down into the invisible 

 part, represented by wider and wider bands, like <I>, \P, XI 

 (composed themselves probably of lines). The heat between 

 these local regions of almost total absorption continues, as I have 

 repeatedly before stated, to be (contrary to the old belief) 

 apparently transmitted with even greater facility than that in 

 the visible spectrum. These bands have growm larger and 

 closer and closer together as we have come down from the 

 visible spectrum below the point where the old map ended, 

 and not far below 5^ they seem to practically coalesce into one 

 almost unlimited cold band. We do not, let it be observed, 

 assert that the absorption is absolutely total even here, and, in 

 fact, there is always a feeble heat to be observed throughout 

 this extent. This, however, the use of the sifting train shows 

 to be largely, at any rate, factitious, but we admit the possi- 

 bility that subsequent research may prove that it is not all so. 



Let us now recur to fig. 2 b, where we shall find below 

 10** the same dependence of the effects upon the season and 

 the hour, as in the part above 5^. 



At 10 K 2 observations made during the autumn showed 

 scarcely the feeblest suspicions of heat, and the same has held 

 good in the very mild weather of the past winter (of 1887) ; 

 but on a few days, when the temperature had fallen below the 

 freezing-point, a notable maximum was found at this point, 

 followed by a minimum at 10^7. The height of this maximum 

 relatively to the principal one in this region at about 13 M ap- 

 pears to be correlated with ihe composition of the air as affected 

 by the temperature. On the coldest day (temperature at noon 

 — 6°*7 C.) the deflection at midday for X= 10^*2 was nearly 

 one-half that at 13^ ; but on other days, when the temperature 

 was near 0° C, the deflection at 10* 1 * 2 did not exceed one- 

 fourth that at 13**, while at temperatures above + 10° C. it 

 was not noticeable. 



It is in the region near 12F to 14**, or over twenty times the 

 length of the visible spectrum below it, that we have found the 

 maximum of the lunar heat-spectrum; and it is here that we first 

 obtained indications of solar radiation corresponding in its great 

 wave-length to this special lunar radiation, but of amounts 

 nonestimable by the means till now employed. I have already 

 spoken of its almost unrecognizably small amount, and a per- 

 haps more vivid apprehension of its extreme minuteness will be 

 gained from the statement that on this graphical construction, 

 on the scale of ordinates used in delineating the curve from 



