PRESIDENT S ADDRESS. XC1X. 



aggregation of these that planets would be formed. Whether 

 this theory will last as long as Laplace's remains to be seen, 

 but theories, even if wrong, are often of great use, and lead 

 to an increase of our knowledge in the investigations they 

 necessitate. Though light-pressure is not a new discovery, 

 it is only lately that it has been measured and taken much 

 into account in the forces which affect the heavenly bodies. 

 It would seem from experiments that the light pressure caused 

 by the sun on a particle of dust of less than one-ten-thousandth 

 of an inch in diameter would be greater than the attraction 

 of gravitation, so that it would tend to recede from the sun 

 into space, though by a dark object, such as a planet, it 

 would be attracted. Gravitation varies, of course, as the cube 

 of a diameter, light-pressure as the square, which accounts for 

 its much greater effect on small bodies than on large ones. 

 This theory opens out quite a new train of considerations 

 in regard to the constitution of the universe, as must any new 

 force which has not hitherto been taken into account. It is 

 suggested that this may be an agent by its action on meteoric or 

 cometary dust in producing that elusive phenomenon, the 

 Zodiacal Light, which is now supposed to be caused by the 

 illumination of such dust by the sun's rays. From observations 

 taken some years ago it appeared that there was a very slight 

 oscillatory movement of the equator backwards and forwards 

 parallel to itself, which could not be accounted for by any 

 known cause ; and observations were instituted of the variation 

 of latitude at several stations, which are still being carried on, 

 and will, it is hoped, lead to some result. Owing to 

 photographic methods and better instruments the discovery 

 of new variable stars and asteroids has become of such common 

 occurrence as to be hardly worth recording here, the latter 

 having been lately found at the rate of about one a week. A 

 reflecting telescope of 100 inches in diameter, the largest ever 

 made, is in course of construction, as a gift to the Mount Wilson 

 observatory. It has been found practicable to produce at a 

 very moderate cost large lenses consisting of a liquid contained 



