MOUNT WILSON OBSERVATORY. 



213 



amounts of the order of the displacements between the p- and n-components 

 in the sun-spot spectra. 



The resulting photographs show an apparent displacement of the central 

 line similar to that found for the p-component; and measurements give results 

 of the same order, though not quite so large, as those found in the spot 

 spectrum. It is evident that in these measures the observer does not set the 

 micrometer wire on the place of maximum absorption, but half-way between 

 the edges of the line. Hence if the edges of two lines actually overlap, the 

 edge of the line on the free side is estimated to lie farther from the center than 

 the edge on the side next to another line. If this is the case in spot spectra 

 the "peak" of the p-component should not be shifted. 



A second test was made with the aid of the Koch microphotometer, thermo- 

 couples being used in place of the photo-electric cells. Photographs were 

 made of the intensity curves of the line X6173 on several strips of the spot 

 spectrum, and the positions of the maxima of these curves were then measured. 

 The depression between the p- and w-components is rather shallow on these 

 photographs, and better results probably could now be obtained because of 

 improvements in the microphotometer. The results, measured directly from 

 the negatives and from two sets of microphotometer curves, are as follows : 



The probable error of both sets of measures is about 0.01 mm. Although 

 the sign of the displacements of the p-component is the same in both cases, 

 the values are reduced so greatly in the case of microphotometric curves that 

 the residual, if of any significance, is probably due to photographic causes. 



The microphotometer has been fitted with an improved thermo-couple, and 

 will be used for additional studies of this character. Meanwhile, the spot 

 plates have been used to give relative measures of the field-strengfchs at dif- 

 ferent levels, and it is hoped that the precision of these determinations can 

 be increased by further work. 



RADIOMETRIC OBSERVATIONS OF THE SUN AND SUN-SPOTS. 



Mr. Pettit and Mr. Nicholson have used their sensitive thermo-couples for 

 measurements of the distribution of energy across the sun's disk and across 

 sun-spots. For this work a self-registering apparatus was designed, the gal- 

 vanometer deflections being recorded as curves on a moving photographic 

 plate in the same way as in the bolometric observations of Abbot and others. 



Drift curves taken across the disk, showing both the total radiation and 

 that for various wave-lengths between 0.4 ju and 2.0 /.t are in satisfactory agree- 

 ment with those already obtained with the bolometer. The study of similar 

 curves of the total radiation across sun-spots and faculae leads to the following 

 conclusions : 



(1) The total radiation of the center of the sun-spots which have been in- 

 vestigated averages 52 per cent of that of the neighboring photosphere. 



