476 proceedings: philosophical society 



does not contribute to the diffraction pattern in the image and hence 

 tends to reduce the contrast and to decrease the sharpness and crispness 

 of the image. This phenomenon arises because diffracted beams which 

 emerge from gratings whose interv^al is of the order of magnitude of 

 half a wave length of light, are sensibly polarized in a plane normal to 

 the lines of the grating, (b) A diaphragm of the rectangular type is 

 recommended for use in the image plane of the eyepiece in order to cut 

 out all light except that from the particular object under examination. 

 This device allows the eye to work at best efficiency because it is not 

 disturbed by extraneous light provided, of course, the field of view 

 covers an angle of lo degrees or more, (c) The importance of a field 

 intensity of illumination approaching that of daylight and best adapted 

 for the eye at any particular time is emphasized; the simplest method 

 for securing this is by means of a substage polarizer in conjunction with 

 the polarizing prism; the polarizer can be rotated and with it the inten- 

 sity of illumination of the field varied. These factors are not important 

 for ordinary' observations because the resolving power there required 

 is not great; but in high-power, critical work they are significant and 

 enable the observer to accomplish with comparative ease that which 

 under other conditions is a matter of difficulty. 



Discussion: At this stage Mr. Karrer's second paper and also that 

 of Mr. Wright were discussed, Messrs. Crittenden, Sosman, 

 Humphreys, and LittlEhales participating in the discussion. 



The last paper of the evening was by Mr. L. A. Bauer, on Further 

 results of observations of the solar eclipse of May 2Q, IQIQ.^ 



A complete series of photographs of the solar eclipse of May 29, 1919, 

 taken by the various astronomical expeditions (Smithsonian Institu- 

 tion at La Paz, Bolivia; Rio de Janeiro Observatory at Sobral, Brazil; 

 and British at Sobral and Isle of Principe, Africa; and the geophysical 

 expeditions of the Department of Terrestrial Magnetism, at Sobral, 

 Brazil, and Cape Palmas, Liberia) was shown. It was shown that the 

 type of the corona was approximately of the intermediate type between 

 that which is seen during years of minimum sunspot activity, when 

 there are great equatorial extensions of the corona, and that shown 

 during years of maximum sunspot activity, when streamers of about 

 the same length extend from the sun in every direction. 



The great red prominence seen at the author's station (Cape Palmas, 

 Liberia), on the southeast limb of the sun turned out to be the largest 

 prominence thus far photographed. At the time of totality it was about 

 100,000 miles high and had a base of about 300,000 miles. With the 

 aid of the data supplied by the Yerkes Observatory it was found that 

 the mean heliographic latitude of the prominence during the eclipse 

 was about 18 degrees south, and on the east limb. With the aid of 

 measurements on the various photographs, the mean heliographic 

 latitude of the pronounced V-rift in the solar corona on the west limb 

 turned out to be about 45° degrees south. Practically diametrically 

 opposite was a less-pronounced rift somewhat in the shape of the letter 

 U. 



1 This Journal 10: 112. 1920. 



