and on those produced by the Earth's Atmosphere. 521 



be communicated transiently to the thinnest plates, merely by 

 an increase of temperature. 



The Iodine vapour acted powerfully upon the middle of the 

 spectrum, and, by an increase of thickness, gradually extended its 

 absorption towards both extremities ; but more rapidly towards 

 the violet one, so as to shew that the final colour must be a homo- 

 geneous red. 



In so far as these two experiments went, they were highly 

 favourable to the speculation which had at first presented itself 

 to me. My attention was now directed to the action of gaseous 

 bodies, and the first trial which I made was with Nitrous Acid 

 Gas. The result of this experiment completely destroyed the 

 hypothesis which had appeared so plausible, and presented me 

 with a phenomenon so extraordinary in its aspectsbearing so 

 strongly on the rival theories of light, extending so widely the 

 resources of the practical optician, and lying so close to the root 

 of atomical science, that I am persuaded it will open up a field of 

 research, which will exhaust the labours of philosophers for cen- 

 turies to come. 



The spectrum of NEWTON, and of all the philosophers of the 

 1 8th century, was a parallelogram of light, with circular ends, in 

 which the seven colours gradually shaded into each other with- 

 out any interruption. The illumination was a maximum in the 

 yellow rays, and the light decayed by insensible degrees towards 

 the red and the violet extremities. In the year 1808, Dr WOL- 

 LASTON conceived the happy idea of examining a beam of light, 

 that passed through an aperture only the twentieth of an inch 

 wide, and he was surprised to see it crossed by seven dark lines, 

 perpendicular to its length. 



About ten or twelve years afterwards, the celebrated optician 

 JOSEPH FRAUNHOFER, without knowing what had been done by 

 Dr WOLLASTON, observed the spectrum formed by the sun's light 

 transmitted through small apertures ; and by applying a telescope 

 behind the prism, he discovered about 600 parallel dark lines 



