114 THE PRESIDENT^ ADDRESS. 



portance attached to that study of Light and the laws of refraction 

 which forms the very foundation-stone on which microscopical 

 science has been reared. Tiuie will not, however, permit us to 

 refer by name to all those early workers who helped to forward 

 our knowledge of the subtle element, but it is sufficient to say that 

 when once the ball was set rolling, by attention being called to the 

 wondrous phenomena associated with Light, it attracted to its study 

 all the ablest philosophers of ancient times, each in turn taking up 

 the marvellous theme, and handing it on, with the additions received 

 at his hands, to be still further elucidated at the hands of others ; 

 thus our knowledge of Light has increased from one step to another 

 since Aristotle first laid the foundation of Optical Science, 2,200 

 years ago. Men in these early days were striving after this know- 

 ledge, and theories became abundant ; thus we find it recorded in 

 the history of this time that Empedocles, 450 B.C., held the 

 opinion that Light consisted of particles emitted from luminous 

 bodies, yet vision was not complete without certain emanations 

 from the eye to the object. Aristotle, 100 years afterwards, called 

 in question this theory, contending that light did not consist of 

 material particles, but was rather an impulse propagated through 

 some immaterial medium ; his teaching, however, is so mixed up 

 with mysticism that one cannot quite determine whether this may or 

 not be considered as the first dawning of the wave theory of Light, 

 which received its more perfect development under the hands of 

 Newton and subsequent scientists. We find that the manufacture 

 of glass had made considerable progress about this period, 450 

 B.C., and the mention of burning glasses by Aristophanes in his 

 comedy of " The Clouds," written about 431 B.C., together with 

 the statement that the Roman fleet before Syracuse, 250, B.C., was 

 burned by Archimides by polished metal specula, although as an his- 

 torical and physical fact open to grave question, yet testifies to the 

 attention of early observers being directed to the refrangibility of 

 the light-rays and their capability of being concentrated in a focus. 

 Although this capability was known, very little practical use was 

 made of it till after the Christian era. Seneca, about A.D. 50, 

 observed the magnifying power possessed by glass globes of water, 

 and Pliny, A.D. 79, describes surgical operations, probably actual 

 cautery, being performed by means of spheres of rock crystal, and 

 he also notices the fact that the rays of the sun coming through 

 a glass globe filled with water, become a source of sufficient 



