115 



THE PRESIDENT S ADDRESS. 



lieat to ignite any inflammable body on which they fall, bnt this 

 power was not connected in the minds of these men with the len- 

 ticular form of the glass. Nevertheless every fact observed created 

 one more link in that chain of our knowledge which spans the 

 wide interval we are considering, and must not be passed by with- 

 out notice. We are still, to a great extent, dealing with an age 

 when sjDeculation was rife, and demonstration had not began ; 

 theory was preceding practice, but Seneca proclaimed in prophetic 

 tones, " The time will come when a future day, and the diligence 

 of a distant age shall bring to light those things which now lie hid; 

 the time will come when our posterity will wonder that we should 

 have been ignorant of things so obvious." But that time is not yet 

 reached in this review of microscopical progress. Claudius 

 Ptolomasus, commonly called Ptolemy, about A.D. 140, finding 

 that his astronomical pursuits necessitated a more accurate know- 

 ledge of the laws governing the refrangibility of the light-rays, 

 set himself the task of working out the refractive indices of a ray 

 passing at different angles from air into water or glass, being led 

 to these calculations by observing that a coin placed at the bottom 

 of a basin in such a position as to be invisible, became visible on 

 pouring in water. He leaves behind him an elaborate collection of 

 these measurements, which furnish the oldest extant example of 

 accurately-conducted physical investigation by experiment. 



A wide gap now intervenes between the researches of Ptolemy 

 and the revival of the subject of magnifying glasses by Alhazen 

 in 1100 ; during this period the subject of Light appears to have 

 lain dormant, but we are getting more into the region of practical 

 optics, for Alhazen observed that objects were magnified when held 

 close to the plane side of the large segment of a sphere of glass. 

 If during this period the practice of optics, if we may so call it, 

 produced no evident result, yet the progress of our knowledge was 

 not stayed, being helped in an eminent degree by the mathematical 

 labours of Euclid, who gained for us a basis of definite calculation 

 which served to forward our knowledge of the proper construction 

 of lenses. 



We know of nothing approaching the character of a lens existing 

 at this period excepting that found by Mr. Layard in his excava- 

 tions at the south-west Palace of Nimroud. As some stress may 

 be laid upon this as proof that some sorts of magnifying glasses 

 were in vogue in the ages of antiquity, I may describe it here, and I 



