180 Mr Newall, On a suggestion [May 25, 



letting it fall on a second slit. We can thus produce monochro- 

 matic images of successive parallel chords of the sun's disc. 



Photographic apparatus is then arranged so that the mono- 

 chromatic images are laid side by side, and a complete photograph 

 of the sun's disc can thus be reconstructed. 



It is clear that there are many ways of bringing about the 

 proper relative motions of the slits, the image of the sun and the 

 photographic plate, but practical experience points to the desira- 

 bility of reducing as far as possible the number of parts between 

 which there is relative motion. 



Mr Hale has in the case of the spectroheliograph of the 

 Kenwood Observatory attached a spectrogenic apparatus, con- 

 sisting of collimator, diffraction grating and camera, to an equa- 

 torial ; and by driving the equatorial at the proper rate he keeps 

 the sun's image stationary relative to the spectrogenic apparatus, 

 the whole being moved relatively to the earth. Then he moves 

 the slit relatively to the collimator, and the second slit relatively 

 to the camera, in which is a fixed photographic plate ; the two 

 slits move at different rates but are connected by carefully designed 

 link-work. 



M. Deslandres has used a heliostat, which does away with the 

 motion of the sun's image relatively to the earth ; and he advocates 

 the relative fixity of slits and spectrogenic apparatus. 



Mr Hale, in using on Mount Etna a spectroheliograph designed 

 by him for the late Mr Cowper Ranyard {Astronomy and Astro- 

 physics, 1894, p. 662), made an advance in arranging that the 

 camera and collimator should be parallel, so that the image of the 

 sun and the photographic plate should be in parallel planes. This 

 method has been adopted by Mr Hale for the spectroheliograph 

 designed for use in connection with the 40-inch Yerkes equatorial. 

 The slits and the spectrogenic apparatus are relatively fixed, and 

 the whole arrangement is bodily moved relatively to the sun's 

 image and the photographic plate. 



In this arrangement the resulting disc of the sun is round and 

 undistorted, and there is no need for the rectification of the 

 distortion which Mr Hale has had to deal with in his earlier 

 investigations. It is perhaps worthy of note that there appears in 

 Mr Hale's photographs of detail on the sun's disc much less per- 

 spective foreshortening of detail near the sun's limb than one would 

 expect to find. 



In another arrangement, namely the " coronagraph " used by 

 Mr Hale at Kenwood, 1893 (Astronomy and Astrophysics, 1894, 

 p. 673), a modification of the Littrow spectroscope was used. But 

 the spectroscope was fixed relatively to the sun's image and to the 

 photographic plate, and the slits were moved, the nature of the 

 case being such that the slits move in opposite directions. 



