346 THE WORLD MACHINE 



motion. Though Newton gave his adhesion, finally, to the 

 " emission " theory, the pages of his Opticks are full of evi- 

 dence to show how often he inclined to the alternative view. 



The theory held its ground, though with increasing difficulty, 

 as one new observation after another was made, for a century 

 or more, and until it came under the keen scrutiny of an English 

 physician, Dr. Thomas Young. The latter showed that by 

 pricking a pair of pin-holes, close together, in a piece of paper, 

 the two little pencils of light which come through, when spread 

 out upon a screen, appear coloured in the interval in which 

 the rays from the two openings cross each other. Very much 

 the same appearance may be noted on very thin soap-bubbles. 

 Black spots appear which, from having been studied by old 

 Sir Isaac, are known as Newton's rings. The fact could only 

 be explained by the idea that the rays in part mutually ex- 

 tinguish each other. Further experiments with very thin plates 

 left no doubt as to the interpretation. 



Evidently, then, light was not a material substance ; it 

 was inconceivable that one little shower of particles could abolish 

 the other in this curious way. If light were immaterial it must 

 be then some form of motion, since there was no other way 

 by which the transmission or propagation of light could be 

 explained. Huyghens, it seemed, had the right of it all along. 

 It was some little time, however, before the conclusions of 

 Young, independently reached a few years later by a young 

 French physicist, Fresnel, could penetrate the understanding 

 of the academic authorities ; it is generally so. This was in 

 the first ten or fifteen years of the new century. A generation 

 later, and in a new generation with fresh eyes to see, a second 

 crucial test, by Foucault, brought conviction. 



Then it was that the investigators began to consider with 

 a larger insight the dark lines which cross the spectral band 

 of sunlight. They had been roughly mapped by Wollaston, a 

 co-worker with Young ; with more care by Fraunhofer, the 

 celebrated telescope-maker of Munich. It had been noted very 

 early that the spectrum of ordinary gaslight, for example, does 

 not show these dark lines ; the gradations of one rainbow colour 

 into another are not interrupted; the spectrum is continuous. 

 Again, it was found that if some of the elementary gases, like 

 those which compose water for example, oxygen or hydrogen 

 were illuminated by an electric spark, their spectral bands 



