of t lie Solar spectrum. 235 



the visible spectrum was about 18 millims. If we imagine the 

 space bordering upon the red end of the spectrum to be divided 

 into strips or zones of 3 millims. in width, then the first, 

 second, third, &c. of these divisions is called by Franz the first, 

 second, third, &c. dark zone. Franz found the following values 

 for the thermal effects in the several divisions of the visible 

 spectrum and in the dark zones : — 



Fifth dark zone 0-83 Red . . 15-11 



The numbers are not the immediate values of the observed 

 deflections of the multiplier, but are obtained by multiplying 

 the mean value of the best observations by a constant factor, 

 the numerical value of which is not given. It is much to be 

 deplored that Franz did not give, at least in part, the direct de- 

 flections he observed; for it is from these alone that a measure 

 can be obtained for the degree of accuracy of his resvilts. 



In consequence of glass absorbing a considerable quantity of 

 dark thermal rays, a curve of thermal intensity constructed from 

 the above numbers by no means represents the distribution of 

 heat in a perfect thermal spectrum ; knowledge of the latter can 

 only be obtained by means of a prism of rock-salt. 



Matters stood essentially in this position when I commenced 

 my experiments upon the thermal efi'ects of the solar spectrum : 

 the results I have obtained furnish the material for the following 

 pages. 



Apparattis, ^c. emploijed in the eorperiments upon the thermal 

 spectrum. 



Before proceeding to the separate series of experiments, it will 

 be necessary to describe the apparatus I employed in performing 

 them, and the manner in which the apparatus was arranged. 



The thermo-battery was erected in a dark room arranged for 

 experiments with solar rays. 



Amongst the several multipliers at my disposal, one which I 

 had myself constructed several years ago, in order to repeat Du 

 Bois Reymond's experiments on the current in muscles, gave in 

 combination with a thermo-battery of forty bismuth-antimony 

 pairs, by far the largest deflections. 



This multiplier consists of 3700 convolutions, which are, how- 

 ever, wound in four separate parcels : each parcel terminates in 

 two screw-clamps, so that the four parcels may be employed 



R2 



