Determination of Coronal Light during Eclipse. 15 



occurring. Further, by using strips of lenses cut out at suitable 

 distances from the axes (fig. 3), images of various colours can be 

 placed side by side upon P, since a slit may be placed in the 

 spectrum opposite each such strip of lens. Incidentally, I may men- 

 tion that investigations into the cause of the variable nature of 

 different flames can be carried out by this plan. 



For solar work, a long collimator appears to be a necessity, but the 

 aperture need not be large. Suppose we determine to have an imacre 

 of the sun on P (fig. 2) of 2 in. diameter, the image on M need not 

 be more than 1 in. at most. For this purpose we must have a colli- 

 mator 10 ft. long. Two lenses of this focal length can be fixed one at 

 each end, and a slit in front of that lens which is presented to the 

 sun's rays. The arrangements followed will be the same as those 

 given for the electric light. There appears no difficulty in producing 

 a monochromatic image of almost any size if the collimator be suffi- 

 ciently long and the face of the prism sufiiciently large to take in the 

 whole of the image cast on it.* 



I have replaced the prism by Hat refraction gratings with most 

 satisfactory results. The gratings I employed had about 6,000 and 

 12,000 lines to the inch. The images were sharply defined, but, of 

 course, weaker than when the prism was employed. For solar work 

 this should not be an objection, since there is plenty of light to work 

 with. 



I show some pictures taken by the plan I have described. For my 

 purpose the images are sufiiciently sharp, although simple uncorrected 

 lenses have been employed. 



* On the Determination of the Photometric Intensity of the 

 Coronal Light during the Solar Eclipse of 16th April, 

 1893." By Captain W. DE W. ABNEY, C.B., D.C.L., F.R.S., 

 and T. E. THORPE, LL.D., F.R.S. Received April 14, 

 Read April 30, 1896. 



(Abstract.) 



In this paper the authors give the results of the measurements of 

 the intensity of the light of the corona, as observed at Fundium in 

 Senegal, on the occasion of the solar eclipse of April 16th, 1893. 

 The methods employed by them were practically identical with 

 those used at Grenada, in the West Indies, during the eclipse 

 of 1886, an account of which is given in the ' Phil. Trans.,' A, 1889, 



* It should be mentioned that to minimise diffraction the slits should be used 

 fairly wide. Hence a long collimator such as described and a good dispersion will 

 be necessary to obtain the best definition of the sun's image. April 30. 



