184 RITMFORD SPECTROHELIOGRAPH. 



soon after Avliicli the instruments of the Kenwood Observatory were 

 removed to the Yerkes Observatory. 



My spectroscopic studies of the sun during the spring and summer 

 of 1891 were not confined to the chromosphere and prominences. It 

 was found that the H and K lines, previously recognized as no less 

 characteristic of the prominences than the hydrogen lines themselves, 

 Avere reversed from dark to bright in regions scattered all over the 

 solar disk. This fact had not escaped the attention of Professor 

 Young, who had long before remarked the presence of these lines in 

 the neighborhood of active sun spots. But the greater delicacy of the 

 photographic processes showed these bright lines to characterize very 

 extensive regions on the sun's surface, not confined to the immediate 

 neighborhood of spots, but scattered throughout the sun-spot zones, 

 and even extending from pole to pole. It w^as noticed from the out- 

 set that these bright regions corresponded closely with the well-known 

 facula% and in my earlier work they were called by this name. It has 

 since become clear, however, that a distinctive term should be adopted, 

 and I now propose the name "■ flocculi " for the regions on the sun's 

 disk which are shown only on photographs made with the spectro- 

 heliograpli. 



The possibility of photographing these bright regions on the sun's 

 disk with the spectroheliograph at once greatly extended the range 

 of that instrument, as it was thus shown to be capable of recording, 

 not only the prominences, Avhich could be observed, though very 

 laboriously, by visual methods, but also extensive and important 

 jihenomena invisible to the eye and not showai on photographs taken 

 in the ordinary manner. Spectroheliographs Avere accordingly 

 adopted for use at other observatories, first by Mr. EA^rshed in Eng- 

 land, and subsequently (in 1893) by M. Deslandres at the Paris Ob- 

 servatory. Both of these spectroscopists introduced modifications 

 and improvements of'tlie instrument, Mr. Evershed constructing a 

 direct-vision spectroheliograph of remarkable simplicity and beauty, 

 and M. Deslandres, Avith a different type of instrument, obtaining 

 photographs of great excellence. 



It had been hoped and expected that the interruption in the daily 

 series of photographs, caused by removal to the Yerkes Observatory^ 

 in 189(j, Avould be of short duration, but unfortunately this did not 

 prove to be the case. The 12-inch refractor, devoted at KeuAVOod 

 entirely to solar Avork, Avas needed at the Yerkes Observatory for 

 general purposes. It therefore became necessary to remove the spec- 

 troheliograph from this telescope, and to modify the mounting in 

 order to adapt it for general obser\ ational Avork. The spectrohelio- 

 graph Avas remodeled for use Avitli the 40-inch refractor as a solar 

 spectroscope, and it was expected that a ucav spectroheliograph, large 

 enough to photograph the 7-inch (17.8 cm.) image at the focus of 



