xviii President's Address 



of the gas from which such light may emanate, or through 

 which it may pass ; and he points out that, comparing 

 any homogeneous light from a heavenly body which can 

 be obtained by help of the prism, with the now well- 

 known phenomena of interference bands, the knowledge of 

 the relations referred to leads us to a means of deducing 

 the pressures prevailing at the points from which this 

 homogeneous light emanates. A knowledge of the pressure 

 of the solar atmosphere is of the highest importance for the 

 elucidation of many great astronomical questions, and 

 especially in such a one as was lately presented to us by 

 the wonderfully close approach of the February comet to 

 the sun's surface, that body having probably grazed or 

 passed through the outer envelopes of the solar atmosphere; 

 and if we may except Mr. Proctor's speculation as to what 

 might have been the result of such an occurrence, this 

 question becomes one of very grave moment to us dwellers 

 on earth. When this comet was first seen here, it had just 

 passed it critical point, and it is fortunate we knew nothing 

 of its close approach to the sun until it was sailing away 

 into space again, as far as we could see, unharmed ; other- 

 wise, being properly prepared by a little sensational 

 writing, with what a lively interest should we have awaited 

 the perihelion passage, pregnant with such possible catas- 

 trophe. At the apparition of this comet in 1843 it 

 approached within 96,000 miles of the sun's surface, and in 

 February last its perihelion distance was probably some- 

 what less. When next it returns, as expected, in 1917 there 

 may some of us be left to experience the result of its closer 

 approach or perhaps its absorption into the sun. 



It is not many years ago since nervous people were made 

 very uncomfortable by graphic descriptions of the effects of 

 a collision between a comet and the earth, but since it has 

 become known that such catastrophes have occurred, with no 

 more result, perhaps, than a beautiful shower of shooting stars 

 or a peculiar foggy state of the atmosphere, the cataclysm of a 



