NATURAL SCIENCE IN SEVENTEENTH CENTURY 265 



It is easy, even for savages, to account for daylight as sunlight, 

 and the corresponding nightlight as moonlight and starlight, but 

 for more highly developed man to explain just what the light is 

 which comes from sun, moon and stars, is not so easy. Obviously, 

 since " luminaries " sun, moon, stars, firebrands and torches 

 produce light which is weaker as distance from the source in- 

 creases, a kind of "emission" theory of light is natural and reason- 

 able. It was even held by the ancients that we see by means of 

 light emitted from our own eyes, and that light is a more or less 

 palpable substance. A similar error was held concerning heat, 

 which until the end of the eighteenth century was generally re- 

 garded as a peculiar material body or substance, "caloric," which 

 when absorbed from other bodies produced a state of heat, and 

 when emitted caused, by its absence, cold. 



How men could have believed for ages that objects are rendered 

 visible by something projected from the eye itself so that the organ 

 of sight was supposed to be analogous to the tentacula of insects, and 

 sight itself a mere species of touch is most puzzling. They seem 

 not till about 350 B.C. to have even raised the question : If this is how 

 we see, Why cannot we see in the dark? or, more simply; What is 

 darkness? The former of these questions seems to have been first 

 put by Aristotle. 



The ancients probably understood that light travels in straight 

 lines, and they must have known something about reflection and 

 refraction of light, for they knew about images in still water, and 

 had mirrors of polished metal, and burning glasses of spherical 

 glass shells, or balls of rock crystal. To Hero of Alexandria we owe 

 the important deduction from the Greek geometers that the course 

 of a reflected ray is the shortest possible (p. 123). 



The perfection of gem cutting among the ancients has also been 

 held to prove their acquaintance with lenses. But it was not 

 until the seventeenth century that modern ideas of light and 

 optics began to be formulated, with the work of Snellius, Descartes 

 and Newton on reflection and refraction, and of Romer on the 

 velocity, of light. 



