The Seventeenth Century 95 



new phenomenon, had been discovered. This property of matter 

 was not mentioned by Bacon and not fully discussed ^ by any pre- 

 vious philosophers. As far as the history of luminescence is con- 

 cerned, the seventeenth century became the Age of Phosphori or 

 Light Bearers. 



Derived from the Greek word for the morning star, phosphor or 

 phosphorus " came to mean any one of the various types of luminous 

 compounds now so commonplace in everyday life. The words " phos- 

 phoric, phosphorical, phosphorescent, phosphoresce, phosphores- 

 cence," etc., are associated in a very general way with any light 

 devoid of heat, for example the " phosphorescence of the sea." 

 Strictly speaking, however, the modern scientific usage restricts phos- 

 phorus to the element, as applied by Elsholz in 1677, while phosphor 

 and phosphorescence apply to any compound, such as the Bononian 

 phosphor, which absorbs light and emits it for some time afterwards 

 in the dark. 



By the end of the seventeenth century, many kinds of phosphores- 

 cent substances were known. The word phosphorescent appeared in 

 prose or poetry, most frequently in connection with the sea (see 

 Chap. V and Chap. XV of this book) . 



Rene Descartes and the French Point of View 



Descartes did not discuss luminescence in any detail but his views 

 on the subject are of special interest, because he has been regarded 

 as the first philosopher since Aristotle to develop a new and unified 

 system of cosmology, and he exerted a tremendous influence on 

 contemporary thought. The Greeks had already exhausted the pos- 

 sibilities regarding the nature of light. Although the various points 

 of view differed in detail, their theories may be placed in four cate- 

 gories. Light was considered to be (1) a stream of particles given 

 off by a visible object (Pythagoras) ; (2) material films or images 

 from the object, flowing to the eye (Democritus and Epicurus) ; (3) 

 particles (Empedocles and Plato) or " emanations " (Euclid) from 

 the eye falling on an object, which is then seen; (4) a quality, i. e, 

 an activity (energeia) of a transparent medium (diaphenes) , not 

 material in nature. Light is not fire, but the presence of a fiery 



" See Chapters I and II for luminous jewels and Chapter VIII for the story (1600) 

 of van Helmont's stone which retained the light of the sun, when placed in the dark. 



^° At first the Bolognian stone was referred to as lapis Solaris, lapis lunaris, lapis 

 lucifer, lapis illuminabilis, lapis phengis (from the Greek, pheggo, to shine or make 

 bright) , later as spongia solis or luna terrestris. In 1640 F. Liceti entitled his great 

 monograph on the Bolognian stone Litheophosphorus or stony light bearer, and the 

 word phosphorus became more and more popular as a designation. 



