June 24, 1897] 



NATURE 



n7 



The instrument was focnssed by photographing the 

 spectra of some of the brighter stars. This is the only 

 satisfactory method of focussing the prismatic camera, 

 as rays from a star fall on the prism under exactly the 

 same conditions as those from the eclipsed sun. If a 

 slit and collimator be employed, identical conditions can 

 only be obtained when the collimator is perfectly achro- 

 matic and absolutely adjusted for parallel rays. 



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Sir 





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I next come to the kind of result obtained by means 

 of the unprecedented optical power employed in 1893, 

 and for this purpose I reproduce two of the beautiful 

 photographs obtained by Mr. Fowler. 



It will be seen that we get more or less complete 

 rings when we are dealing with an extended arc of the 

 chromosphere, or lines of dots when any small part of 

 it is being subjected to a disturbance which increases 



NO. 1443, VOL. 56] 



the temperature, and possibly the number of thedifferen 

 vapours present. 



The interpretation of these photographs brings us ii> 

 preserice of many interesting and, at the same time, 

 complicated problems. I cannot, however, refer to then\ 



15 





here. I have set them out at length in the P/n7. Trans. 

 (vol. for 1896 A, p. 551). 



The instrument so successfully employed by Mr. 

 Fowler was not the only one used in 1893. I had 

 been able to equip Mr. Shackleton, one of the com- 

 puters attached to the Solar Physics Observatory, 



