2 



A POPULAR ACCOUNT 



Aristotle devoted considerable atten- 

 tion to the phenomena of rainbows, par- 

 helia, halos, &c. He observed these 

 with accuracy, and ascribed the rain- 

 bow to the imperfect reflection of the 

 sun's rays by drops of rain. Some of 

 the properties of concave and convex 

 spherical mirrors appear to have been 

 taught in the Alexandrine school, and 

 are explained in a treatise on optics 

 contained in the works of Euclid. 



(3.) The phenomena of refraction not 

 being of so striking a kind, or of such 

 frequent occurrence as those of reflec- 

 tion, do not seem to have been noticed 

 until a later period. The power of re- 

 fractors to collect the sun's rays to a 

 focus, so as to burn any substance ex- 

 posed to their influence, was, however, 

 long known. Burning refractors are 

 very distinctly described in Aristophanes 

 comedy of The Clouds, and Aristotle 

 observed the broken appearance of a 

 stick held obliquely in water, and at- 

 tempted to account for it. In the first 

 century of the Christian era several 

 optical phenomena were investigated by 

 Seneca, and among others he observed 

 that writing viewed through a glass 

 bottle filled with water was magnified. 

 This was, probably, the first discovery 

 of the magnifying power of a refracting 

 medium, bounded by convex surfaces. 

 Seneca also noticed the colours pro- 

 duced by a prism or angular piece of 

 glass, and observed that they were si- 

 milar to those of the rainbow. 



(4.) The most distinguished among 

 the ancients for discovery in optical 

 science was Claudius Ptolemy, the 

 celebrated astronomer of Alexandria, 

 who flourished in the second century of 

 the C hristian era. This philosopher was 

 the first who observed, with any degree 

 of scientific precision, the phenomena 

 of refraction. He appears to have sys- 

 tematized, improved, and imparted a 

 greater degree of accuracy to all that 

 was previously known concerning the 

 reflection of light at plane and curved 

 surfaces, but he must justly be con- 

 sidered as the first and exclusive dis- 

 coverer of the principal phenomena of 

 refraction or dioptrics. 



Ptolemy observed that if a visible 

 object, a piece of money for example, be 

 laid on the bottom of a vessel, and the 

 eye so placed that the edge of the vessel 

 just intercepts the view of the object, 

 upon filling the vessel with water, the 

 object will seem to be gradually raised, 

 until at length it comes distinctly into 



view. This fact led him directly to the 

 true nature of refraction, and shewed 

 that the visual ray in passing out of the 

 water at the surface, was bent towards 

 the eye of the spectator, so as to make 

 the object on the bottom of the vessel 

 appear higher in the water than its 

 real position. He invented an instru- 

 ment to measure the deflection of the 

 ray in passing from the water into air. 

 This instrument consisted of a circle, 

 carrying two sights on its graduated 

 rim, and a third sight at the centre. 

 The circle was immersed in the water, 

 with its plane perpendicular to the sur- 

 face, and so that the surface of the 

 water coincided with one of its diameters, 

 and that one of the sights on the rim was 

 above and the other below the surface. 

 The eye being placed at the sight above 

 the surface, the sight below the surface 

 was moved upon the rim, until the three 

 sights appeared to lie in the same 

 straight line. The distances of the sights 

 on the rim, from the highest and lowest 

 points of the circle, then shewed the 

 angles of refraction and incidence. 



With this instrument Ptolemy ob- 

 served and calculated the refractions 

 corresponding to every ten degrees of 

 incidence in the quadrant, the refraction 

 being made between air and water. He 

 also measured and calculated the re- 

 fractions between air and glass and 

 water and glass, by cutting the glass 

 into a semi-cylinder of the same dia- 

 meter as the graduated circle, and ap- 

 plying the semicircle to the end of the 

 semi-cylinder. 



The results of these experiments 

 showed that the deflection of the ray 

 between glass and water was less than 

 in either of the cases between water or 

 glass and air. From this he was led to 

 conclude that the difference of the den- 

 sities of the media was the cause of re- 

 fraction, since water and glass differed 

 less in density than air and glass, or air 

 and water. This fortunate observation 

 suggested the probability that light from 

 celestial objects, in passing into our at- 

 mosphere from the surrounding me- 

 dium, whatever it be, might suffer a 

 deflection, the consequence of which 

 would be that our view of the whole 

 face of the heavens must be distorted ; 

 objects appearing variously removed 

 from their true places, according to their 

 various positions with respect to the 

 highest point or the zenith. The only 

 point which would not be removed from 

 its true place under such circumstances, 



