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INTERNATIONAL ASTRONOMICAL UNION Proc. N. A. S. 
the study of many questions, such as the possible connection between solar 
eruptions and terrestrial magnetic storms. 
Most of the telescopes hitherto used for this work give a very small 
solar image, while the spectroheliographs employed with them are in very 
few cases of sufficient dispersion to permit satisfactory photographs to be 
taken with the Ha line. As the hydrogen flocculi, for most purposes, are 
of much greater interest and importance than the calcium flocculi, the 
object of this report is to indicate the general nature of the instrumental 
equipment needed to record them in such a way as to render the resulting 
photographs fairly comparable. 
The chief instrumental requirements are: 
1. A telescope giving a large solar image, not less than 10 cm. and pref- 
erably from 15 to 20 cm. in diameter. This large scale is needed in order 
that the minute structure defining the fields of force of the hydrogen flocculi 
may be clearly recorded. 
2. A spectroheliograph of sufficient dispersion to permit the hydrogen 
flocculi to be photographed with light confined to the central part of 
the Ha line. A linear dispersion of 1 mm. = 3.5 A will serve very well. 
It is desirable, but not essential, that this spectroheliograph should be 
of sufficient aperture to permit the entire solar image to be photographed 
at once. If this is not feasible, a second objective, giving a smaller solar 
image (at least 5 cm. in diameter), should be so attached to the telescope 
that it can be readily substituted for the objective of greater focal length. 
3. It is important that spectroheliograms intended for comparative 
studies of eruptive phenomena and changes in the structure of the hydrogen 
flocculi should not differ greatly, either in the size of the solar image or 
in the dispersion of the spectroheliographs with which they are taken. 
For this reason it is desirable that the optical constants of the instruments 
used at cooperating observatories should be as nearly alike as possible. 
4. It is desirable that the spectroheliographs should be adaptable for 
the photography of the calcium flocculi with K2 and with K3. It would 
also be advantageous to design them for use as spectroregistreurs des 
vitesses (Deslandres) and for the study of the magnetic fields in sun-spots, 
in case spectrographs especially designed for these purposes are not avail- 
able . 
A final word should be said regarding the necessity for good atmospheric 
conditions, to permit the sun to be photographed -on the largest possible 
number of days and to afford the perfection of definition required to reveal 
the minute details of the flocculi. 
George B. Hale. 
REPORT OF THE COMMITTEE ON ECLIPSES 
(Abstract) 
The Committee believes that it might be of assistance to astronomy 
along the following lines : 
