1896.] for a form of specbroheliograph. 183 



direct vision as defined on page 181 is horizontal and coincides 

 in direction with the axis of the horizontal beam of sunlight ; 

 (c) the slits are vertical. This pendulum is to be set swinging 

 with an amplitude slightly greater than the diameter of the sun's 

 image. 



4. A photographic plate is to be set either all but in contact 

 with the second (monochromatic) slit, or else considerably further 

 back, with a lens placed between it and the second slit in such 

 a way as to form an enlarged image of the second slit on the 

 plate. The lens and photographic plate are to be fixed relatively 

 to the earth and consequently also to the image of the sun. 



The resulting photographs will shew the average appearance of 

 the disc during the whole exposure. The central parts of the 

 sun's disc would get less exposure in a given interval of swinging 

 than those parts of the borders of the disc at the ends of a hori- 

 zontal diameter of the image : but this would be no disadvantage 

 if one considers the difference in brightness between the centre 

 and the limb. The relation between the exposure at the centre 

 and near the limb might also be controlled in their small limits by 

 altering the amplitude of the swing. Loss of definition would 

 arise either because of atmospheric disturbance, which would affect 

 the whole disc, or because of violent motions on the sun's surface, 

 which would affect only restricted regions on the disc. The 

 motions appear however to be seldom so great as to be likely 

 to affect photographs taken with comparatively short exposures, 

 and on such occasions local loss of definition would indicate the 

 disturbed regions. 



I have hesitated to commit this suggestion to paper without 

 giving it a practical trial ; my only excuse is that I am in hopes 

 that some one may be induced to give the method a trial earlier 

 than I see any prospect of being able to do myself. 



(3) On the Period of the Earth's Free Eulerian Precession. 

 By J. Larmor, M.A., F.R.S., St John's College. 



1. The main object of this note is to state a principle which 

 allows us to estimate the effect of elastic yielding of a rotating 

 solid on the period and character of the free precession of its 

 axis of rotation. It has been suggested by the recent papers of 

 Professor Newcomb and Mr Hough 1 , in which the discussion is 

 chiefly confined to the case of an incompressible homogeneous 

 spheroid. 



1 S. Newcomb, Monthly Notices R.A.S., 1892; S. S. Hough, "The Rotation of an 

 Elastic Spheroid," Phil. Trans., 1896. 



