Page:Proceedings of the Royal Society of London Vol 60.djvu/23

Rh 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 mention 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 image 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 m ust have a collimator 10 ft. long. Two lenses of this focal length can be fixed one at each end, and a slit in front of th at 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 sufficiently long and the face of the prism sufficiently large to take in the whole of the image cast on it.

I have replaced the prism by fiat 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 sufficiently sharp, although simple uncorrected lenses have been employed.

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 W est Indies, during the eclipse of 1886, an account of which is given in the ‘ Phil. Trans.,’ A, 1889,