26o POPULAR SCIENCE MONTHLY. 



of years the moon will be a full moon in the sky, but a new moon in 

 the calendar. Nothing could be neater than the presentation of this 

 dilemma. 



Bacon understood the theory of vision, the anatomy of the eye and 

 much of the physiology of perception, as well as the theory of lenses 

 and of the simple microscope. He did not combine two lenses to make 

 a telescope, but he was on the high road to it. His works on alchemy 

 were undertaken in the same scientific spirit, though in the infancy 

 of chemistry they led to few results of value. Gunpowder he knew, 

 very likely through the Arabs or the Greeks of Constantinople, who 

 had it from the east. The children of his time played with it, he says. 



No better example of the experimental method imagined and ex- 

 tolled by Bacon can be given than an analysis of his brilliant demon- 

 stration of the nature of the rainbow. Let the experimenter, he says, 

 first consider the cases in which he finds the same colors; as in the 

 hexagonal prisms of Iceland spar, for example. By looking into these 

 he will see the rainbow hues. Many think that these arise from some 

 special virtue of the stones, or from their hexagonal figure. Let the 

 experimenter therefore go on and he shall find the same colors in other 

 stones of other shapes, as well as in the drops of water dashed from 

 oars in the sunshine, and the like. All these are instances like the 

 phenomenon of rainbow colors. With regard to the form of the bow 

 he is still more precise. He bids us measure the altitude of the bow 

 and of the sun and note that the center of the bow is exactly opposite 

 to the sun. He explains its circular form — its independence of the 

 form of the raincloud — its moving when we move — its flying when we 

 follow — by its consisting of reflections from a vast multitude of minute 

 drops of rain. In the iris shown by the spray of a waterfall we may 

 see the whole circle. In the sky the plane of the horizon comes in to 

 interfere. Each drop of rain in the cloud is to be regarded as a 

 spherical mirror. 



His views of the nature of force are expansions of those held 

 vaguely by Democritus and Lucretius. A body is a center of force 

 from which energies radiate in every direction. Every action is ac- 

 companied by a reaction, and there is an interchange of force between 

 all bodies of the universe. The propagation of force, of light for ex- 

 ample, requires time. "There is no substance on which the action 

 involved in the passage of a ray may not produce a change. Thus it 

 is that rays of heat or sound penetrate the walls of a vessel of gold or 

 brass. In any case, there are many dense bodies which altogether in- 

 terfere with the visual and otlier senses of man so that rays cannot 

 pass so as to produce an effect on human sense and yet, nevertheless, 

 rays do really pass, though without our being aware of it." These pre- 

 cise statements were mere words to his contemporaries and could not 



