SECT. XXIV. PRISMATIC AND DEFRACTED SPECTRA. 223 



produce phosphorescent light, which is extinguished by the 

 least refrangible luminous rays. According to Mr. Hunt's ex- 

 periment, the hot rays condense mercurial vapour on a polished 

 metallic plate, while the luminous rays prevent its formation. 

 Electricity is excited by the chemical rays, while the parathermic 

 are found in the less refrangible rays alone. Each of the spectra 

 is crossed by coloured and rayless lines peculiar to itself, and 

 these are traversed at right angles by innumerable dark lines of 

 various breadths, the whole forming an inexpressibly wonderful 

 and glorious creation. 



The arrangement varies a little according to the material of 

 the prism and the manner of producing the spectrum, as in that 

 obtained by Professor Draper from diffracted light. It was 

 formed by a beam diffracted by passing through a netting of 

 fine wire, or by reflection from a polished surface of steel, having 

 fine parallel lines drawn on it. This diffracted spectrum is 

 divided into two equal parts in the centre of the yellow; and 

 as in the prismatic spectrum, one half is antagonist to the other 

 half, the ,red or negative end undoing what the positive or 

 violet end has done. The centre of the yellow is the hottest 

 part, and the heat decreases to both extremities. A line of cold 

 is supposed to exist on this spectrum answering to Fraimhofer's 

 dark line H. 



The undulations of the ethereal medium which constitute a 

 sunbeam must be infinitely varied, each influence having a 

 vibration peculiar to itself. Those of light are certainly trans- 

 verse to the direction of the ray ; while Professor Draper believes 

 that those of heat are normal, that is, in the direction of the ray, 

 like those of sound. A doubt exists whether the vibrations of 

 polarised light are perpendicular to the plane of polarisation or 

 in that plane. Professor Stokes of Cambridge has come to the 

 conclusion, both from the diffracted spectrum and theory, that 

 they are perpendicular to the plane of polarisation, but M. Holtz- 

 mann is of opinion that they are in that plane, so the subject 

 is still open to discussion. 



