436 The Bev. Walter Sidgreavcs [Jan. 22, 



and micrometers, for the prosecution of his studies in solar physics, 

 and a well equipped prismatic instrument for the stars. 



With the first of these I had hopes of carrying on Fr. Perry's 

 work on spot-spectra, with the acknowledged advantage of the grating 

 dispersion. It was a promising field for work with an excellent 

 instrument ; and my disappointment was great when I turned from 

 the text-books to the instrument, and tried to see what others had 

 seen and sketched. I photographed the spectra of macy spots in 

 the rich year of 1893, only to learn more clearly that any conclusions 

 based on the spectra of sun-spots must remain outside my sphere of 

 influence. My last attempt was an imitation of a spot-spectrum for 

 comparison with the real thing. A thread drawn across the slit, and 

 kept in a state of irregular vibration, gave me all that could be 

 desired, except the much-needed help to read the real spectrum 

 correctly. The imitation (projection on screen) seems to me perfect; 

 so perfect that I am unable, by the widening of the lines alone, to say 

 which of the two is the counterfeit. I know them only by the diamond 

 marks on the negatives, and by the companion smaller spots on the 

 unpretended photograph. Others can differentiate between widened 

 and unaffected lines, and they see all the lines widened in the 

 imitation. This is as it should be ; for the widening is the result of 

 the well-known photographic effect of light encroaching on darkness: 

 the dark line is thinned by the neighbouring brightness of the con- 

 tinuous spectrum, off the spot, and hardly at all in its shaded band. 

 Allowance, therefore, had to be made for this photographic effect, 

 and the difference between the real and the pretended spectrum was 

 so small that nothing remained on my photographs to deal with. 

 This experience cast a doubt over the reality of spot-spectra, 

 which has only quite recently been removed by the transparency 

 now on the lantern, it is from a photograph taken at the Yerkes 

 Observatory, with a solar image of 7 inches diameter, by the great 

 telescope. The spectrum shows widened lines, which I do not think 

 could bo imitated; and Professor Hale is probably right in saying 

 that a much larger image of the sun is wanted than has been found 

 possible with the light supplied by the small heliostat at Stonyhurst. 



On the whole, you will not be surprised that I gave up the 

 spectroscopic study of the sun-spot problem, and have since then left 

 it to better eyes. 



But in other ways the solar spectrograph has written its teaching 

 in clearer characters. It has given the answer to a question modestly 

 expressed in the discourse already mentioned: "Might not the last- 

 mentioned observations suggest the question, whether absorbent 

 vapours may not sometimes be cast up from the seething mass 

 beneath, although a down-rush be the prevailing feature of a sun- 

 spot?" The photograph projected on the screen is the spectrum of 

 a spot, in the H K region ; and if I read correctly the language of 

 light, it says, by these bright reversals of calcium vaj)our, that the 

 vapour has been cast up from the hotter lower regions high into the 



