1903.] The Cosmical Function of the Green Plant. 461 



fixed bodies, being heated, emit light so long as they continue suffi- 

 ciently hot ; and light mutually stops in bodies as often as its rays strike 

 upon their parts, as we shewed above." And some lines further : " The 

 changing of bodies into light, and light into bodies, is very conformable 

 to the course of Nature, which seems delighted with transmutations." 

 And further on : " among such various and strange transmutations, 

 why may not Nature change bodies into light and light into bodies ? " 



If after having read these passages we turn to a micro-spectrum and 

 see the chlorophyll-corpuscles, retaining their natural colour and trans- 

 parency in the green and yellow rays, turn black as jet on being 

 removed to certain regions in the red and blue, we may be sure, after 

 all that has been said, that here we actually assist at that mysterious 

 " transmutation of light into bodies " which makes of this black speck 

 of matter the real link uniting that glorious outburst of energy in 

 our central star with all the manifold manifestations of life in our 

 planet. 



It was the prism which more than two centuries ago revealed the 

 inner nature of the sunbeam. It is the prism again which continues 

 in our President's, in Sir Norman Lockyer's, and other able hands to 

 unravel the mysteries of the origin of this sunbeam, and I am confident 

 that it will still be the prism that will some day fully disclose the 

 ultimate fate of this sunbeam on this our earth : "its transmutation 

 into bodies." 



Thus we arrive at the final conclusion that in this book of ' Opticks ' 

 we not only find the first and broadest statement of our problem 

 in its actual state, but likewise the surest means towards its probable 

 solution in the future. Little did Jonathan Swift suspect, when 

 writing his envenomed satire on the Eoyal Society, that that which he 

 took to be the vagaries of a madman was prophetic of Newton's 

 immortal genius. 



DESCRIPTION OF PLATES. 



PLATE 20. 



Apparatus for experimental research in coloured light of normal intensity (see 

 page 436). 



A. The spectrum is recomposed into two or three vertical bands lying side 



by side. 



B. The spectrum is recomposed into two bands, superposed one over the 



other. 



PLATE 21. 



Phytoactinometer apparatus for estimating the quantity of radiant energy 

 absorbed by the chlorophyll of leaves (see p. 448). 



PLATE 22. 

 Cosine photometer, for experimental research (see p. 450). 



VOL. LXXII. 



