SOLAR OBSERVATORY. 139 



In the measurement of heliographic positions, the heliomicrometer has 

 continued to give excellent results, described elsewhere in this report. In 

 the comparative study of the minute details of the flocculi, experience has 

 shown that the most satisfactory results can not be expected until the 30-foot 

 spectroheliograph becomes available for this work. Small atmospheric dis- 

 turbances, though hardly sufficient to affect the sharpness of the photographs, 

 produce slight irregular displacements of details which are rendered appre- 

 ciable by the stereo-comparator when the monocular attachment is employed. 

 While these displacements do not interfere with the general comparison of 

 hydrogen and calcium flocculi, for example, they are objectionable when the 

 relative positions of minute details are to be studied micrometrically. Such 

 difficulties should entirely disappear when the 30-foot spectroheliograph of 

 the tower telescope is employed, since in this instrument two photographs 

 can be taken simultaneously in different lines. 



The construction of the 30-foot spectroheliograph of the tower telescope 

 has been postponed until the difficulties encountered in securing suitable 

 prisms can be overcome. Experiments made by Zeiss and Mantois have 

 failed to yield large prisms of glass sufficiently homogeneous to give good 

 definition. It has therefore become necessary to try experiments with fluid 

 prisms, which promise some hope of ultimate success. 



Meanwhile the 30-foot Littrow spectrograph of the tower telescope has 

 been provided with an attachment which will permit it to be used as a spec- 

 troheliograph. This was not done until experiments with a temporary 

 spectroheliograph, described in Contributions from the Solar Observatory 

 No. 7, proved that an auto-collimating instrument, containing a grating, was 

 capable of yielding good results with the narrower dark lines of the spectrum. 



Hitherto all discussions of spectroheliographic results have been based 

 on the hypothesis that the flocculi are masses of luminous and absorbing 

 vapors lying above the photosphere. While this hypothesis seems to be in 

 good agreement with the observed phenomena, we must not overlook the 

 possibility that it does not correctly represent the facts. Monochromatic 

 photographs of the sun are made under conditions which are most favorable 

 for the registration of any effects that may be due to anomalous dispersion. 

 In the experiments made on Mount Wilson by Professor Julius, and de- 

 scribed elsewhere in this report, the 5-foot spectroheliograph was employed 

 to photograph the sodium vapor within a tube through which sunlight was 

 transmitted. Slight inequalities in density, produced by blowing a current 

 of air through a tube surrounded by the vapor, were easily recorded by the 

 spectroheliograph. In our ordinary assumptions regarding the condition of 

 the calcium vapor in the sun we consider density gradients, exceeding those 

 in Professor Julius's sodium tube, to be present. It is, therefore, very prob- 

 able that anomalous dispersion phenomena may affect, if they do not alto- 



