182 Mr Neivall, On a suggestion [May 25, 



purposes, an eyepiece being attached in place of the slit F. It 

 might be preferable in such a case to have the slits parallel to 

 the axis of the collimator, a corresponding change being made 

 in the position of the grating.) For spectroscopic purposes an 

 achromatic object-glass would be convenient, but for the use of 

 the instrument in a spectroheliograph a single lens is all that 

 is needed. A suitable small stop must be set on the inner 

 surface of the lens near its centre to cut off stray reflected light. 



So far as I am aware, it has been the custom in all forms 

 of spectroheliograph to move the slit slowly across the sun's disc. 



Accurately equable motion is needed in order to avoid streaks 

 in the resulting photographs — the streaks here referred to are 

 those parallel to the slit, not those which are perpendicular to 

 the slit and are due to dust on the slit — and even when the 

 motion is equable, the effect of a cloud passing in front of the 

 sun will be to diminish the density of the photograph only in 

 that part of the disc over which the slit moves during the passage 

 of the cloud. It often happens that the best definition in solar 

 observations is obtained when there is considerable haze and even 

 fog, which is necessarily of varying opacity. 



The necessary exposure for the whole disc may be attained 

 either by moving the slit slowly once across the disc or by letting 

 the slit pass comparatively quickly backwards and forwards over 

 it often enough to get the necessary total exposure, the image 

 of the sun being kept stationary whichever method is adopted. 

 Mr Hale has adopted the first method, and I am not aware that 

 the second method has been either tried or suggested. The 

 oscillatory motion might be given to the slits and connected 

 spectrogenic apparatus by properly suspending the apparatus 

 and allowing it to swing like a pendulum. This method involves 

 the use of a fixed beam of sunlight. 



The suggestion which I have to make may be summarized as 

 follows : — 



1. A ccelostat is to be used to throw a horizontal beam of 

 sunlight. 



2. An object-glass is to be inserted in the beam so as to 

 form a fixed image of the sun — 'JixecV in the sense connected with 

 the ccelostat as opposed to the heliostat. 



3. The spectrogenic apparatus with fixed slits described and 

 illustrated above is to be suspended like the bob of some forms 

 of ballistic pendulums, and in such manner that (a) the slit A 

 receives the image of the sun upon it, and bisects the circular 

 disc of the sun when the apparatus is at rest; (b) the line of 



