324 POPULAR LECTURES AND ADDRESSES. 



must be noticed that Newton did not understand 

 what we call a homogeneous spectrum ; he did 

 not produce it, and does not point out in his 

 writings the conditions for producing it. With an 

 exceedingly fine line of light we can bring it 

 out as in sunlight, like this upper picture red, 

 orange, yellow, green, blue, indigo, and violet, 

 according to Newton's nomenclature. Newton 

 never used a narrow beam of light, and so could 

 not have had a homogeneous spectrum. 



This is a diagram painted on glass and showing 

 the colours as we know them. It would take 

 two or three hours if I were to explain the subject 

 of spectrum analysis to-night. We must tear our- 

 selves away from it. I will just read out to you 

 the wave-lengths corresponding to the different 

 positions in the sun's spectrum of certain dark 

 lines commonly called " Fraunhofer's lines." I 

 will take as a unit the one hundred thousandth 

 of a centimetre. A centimetre is '4 of an inch ; 

 it is a rather small half an inch. I take the 

 thousandth of a centimetre and the hundredth 

 of that as a unit At the red end of the spectrum 



