150 Prof. S. P. Langley on 



In previous communications I have given a representation 

 of the solar-heat spectrum terminating near 2^*7 or 2^*8, and 

 I have stated that while there were feeble indications of solar 

 heat below this point, yet that the solar radiation beyond 

 seemed sensibly cut off, as though below this were a nearly 

 unlimited cold band. I do not mean then, in saying that 

 solar heat sensibly ceases below this point, to say that abso- 

 lutely none can exist, but that none, at any rate, does exist 

 sensible to the delicate apparatus with which these first deter- 

 minations were made, and that none in any case exists of an 

 order in the least comparable with the smaller portions of that 

 already described. 



The reader will gather a more clear conception of the diffi- 

 culty of decision and of the almost infinitesimal amount of this 

 solar heat, if it exist, by looking at Plate IV. fig. 1 , in connection 

 with the statement that if there be any solar heat at 4^ the 

 highest ordinate representing it, on the same scale as that 

 shown on the left of the Plate, would at any rate not occupy 

 the thickness of the horizontal line which represents the axis 

 of abscissae. However, since we are rather inclined to admit, 

 from our final experiments with our latest and most sensitive 

 apparatus, that heat of some kind reappears here (near 4^), 

 whether from the atmosphere of the sun, or elsewhere, insen- 

 sible to the most delicate thermopile, and in any case, if it be 

 real, almost infinitesimal in degree, or of the same order of 

 intensity with that in the lunar spectrum,, our statement that 

 no sensible solar heat exists in this part of the spectrum must 

 be taken under this qualification. 



New Apparatus. 



The apparatus for the determination of wave-lengths in 

 connection with the flint-glass prism has been already 

 described*. The following is the same in principle, with 

 certain changes to adapt it either to the solar or electric 

 heat. Let S x be the first slit (see Plate V). For solar 

 heat it has doubly-moving jaws, controlled by a micro- 

 meter-screw, while in the case of the electric arc we use a 

 special form of slit surrounded by water, to be directly de- 

 scribed. G is the large concave grating. The massive beam A 

 carries the large rock-salt train S 2 , L l3 P, L 2 , B. G is fixed 

 at the extremity of the beam, so that its collimating axis 

 coincides with that of Lj; and by means of an automatic ap- 

 paratus, not shown here, the slits Si S 2 are caused to lie always 

 in the same straight line at right angles to G Si. Under 



* This Journal, March 1884, p. 194. 



