The Eighteenth Century 177 



LEONARD EULER 



Leonard Euler's Letters to a German Princess published in French 

 in 1768-1772, and in English in 1795, is practically a textbook of 

 physics. It contains an excellent account of his earlier (1746) theory 

 of light as a wave motion, and reflection of light as an actual emis- 

 sion of light, when the impinging beam sets up sympathetic vibra- 

 tions in the opaque body. The different colors of the object repre- 

 sented different frequencies of vibration. The book contains his 

 explanation of phosphorescence, depending on vibrations in the 

 phosphor, set up by light (more fully described in Chapter VIII) , 

 and remarks on the electric light, which he regarded as an indica- 

 tion of the " electric atmosphere " around a charged body. 



Euler did not undertake an explanation of the luminescence of 

 animals, or shining wood, but he did consider the aurora boreal is. 

 This light he attributed to particles in the earth's atmosphere rather 

 than particles from the sun (see Chapter VII) . 



GEORGE ADAMS 



Even toward the end of the century there was little change in the 

 conceptions regarding " phosphoric bodies," as indicated in the 

 treatise on physics by George Adams (1750-1795) , an optician and 

 instrument maker of London, with a broad interest in natural phe- 

 nomena. His book. Lectures on Natural and Experimental Phi- 

 losophy, considered in it's present state of improvement, describing 

 in a familiar and easy manner, the principal phenomena of nature; 

 and showing, that they all cooperate in displaying the Goodness, 

 Wisdom, and Power of God, was published at London in 1794. 



It is quite apparent where Adams obtained his information. In 

 Vol. II, the various types of luminescence are grouped according to 

 the classification of Beccari, which Adams quoted almost verbatim. 

 There were the (1) spontaneous linninescences and (2) the excited 

 luminescences, of which the mode of excitation might be 



attrition, agitation, heat, the free admission of air, and being exposed 

 to the external light. 



Bodies of every kind become phosphori by attrition, provided they can 

 bear that force of friction which is sufficent to produce the reluctant light 

 that is hid in their substances; agitation agrees mostly with liquid sub- 

 stances, as sea-water 



The emerald phosphorus, and many gems, and amongst these not a few 

 diamonds, the lapis lazuli, and a great part of the mountain crystals, 

 become phosphoric by the application of heat. 



