1883.J On the Solar Spectrum and Solar Temperature. 331 



ducting material, and the jacket of the pile was kept full of water, 

 the whole being* enclosed to form part of the camera by means 

 of a thick covering. At each movement of the pile at least three 

 readings of the galvanometer were taken. In each series of observa- 

 tions, and for each water spectrum examined, at least six series were 

 made. 



We believe that the results we put forward are fairly representative 

 quantitatively, and without doubt the positions of maxima and minima 

 maybe relied onto one-eighth turn of the screw. At the same time it 

 must be remembered that the breadth of the slit of the thermopile is 

 a measurable quantity, and one which cannot be neglected, but its 

 existence is a necessary evil. For this reason the positions of bands 

 are more exact in the photographs than in the thermograms, and any 

 small discrepancy in position should be discounted in favour of the 

 photograph. When using the solar radiation, which is four times 

 more intense than that of the crater of the electric light, and when 

 both slits have been narrowed, the absorptions are sure to be more 

 intensely marked than when a wider slit is used, the reason of which 

 is obvious. Langley employed a spectrum which was of about the 

 same purity as ours ("American Journal of Science," vol. 25, p. 169, 

 1883, and " Phil. Mag.," vol. 15, p. 153, 1883), bat the width of his 

 bolometer strip being about one-fifth that of the jaws of the thermo- 

 pile slit, and his spectrum longer, the absorptions are shown with 

 greater strength than they are in our curves, whilst in our photo- 

 graphs the absorptions are even more marked. 



Our first result was the thermogram of the crater of the positive 

 pole, and this is shown in Curve I, Diagram I. This is the mean of a 

 dozen curves which all lie very closely on this mean. One thing we 

 learn from it is that the temperature of the crater is always very 

 uniform with any carbon, and is what we might expect when we 

 consider that it is the temperature at which carbon vaporises. In 

 comparing it with Tyndall's curve given in " Heat as a Mode of 

 Motion," we are struck by the difference exhibited in the two near 

 the apices ; this difference, however, is explicable as will be seen 

 further on, when it is remembered that his source of radiation was 

 one composed of various temperatures. Next, in order to ascertain 

 the amount of absorption due to different thicknesses of water, it was 

 found necessary to employ cells with glass sides, and, unfortunately, 

 not being in possession of parallel glass of the same material as the 

 prisms, it became necessary to take the absorption curve of the cells. 

 This is found in Curve II. Water was then placed in the cells before 

 the slit, and the readings taken. Curves VI, VII, and VIII are the 

 water curves corrected for the cell-absorption, and they were con- 

 structed to enable a comparison to be made with the curve of the 

 solar spectrum. 



