302 NINETEENTH CENTURY. pt. hi. 



CHAPTER XXXII. 



SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). 



Discoveries concerning Light made in the Nineteenth Century — Birth 

 and History of Dr. Young — He explains the Interference of Light 

 — Cause of Prismatic Colours in a Shadow — And in a Soap-bubble 

 — Malus discovers the Polarization of Light caused by Reflection — 

 Birth and History of Fresnel — Polarization of Light explained by 

 Young and Fresnel — Complex Vibrations of a Ray of Light — How 

 these Waves are reduced to two separate Planes in passing through 

 Iceland Spar — Sir David Brewster and M. Biot explain the colours 

 produced by Polarization. 



We must now go back to the history of Light, which we left, as 

 you will remember, at the end of the seventeenth century, at 

 the point which Newton and Huyghens had reached. During 

 the whole of the eighteenth century very little was learnt about 

 this science, and it remained for the men of our own time to 

 make the next step and to discover the grand laws of light 

 which we must now consider. It will be best to divide our 

 subject into two parts — ist. The discoveries which have led 

 to a true Theory of Light. These are very difficult to under- 

 stand, and you must not expect to gain more than a slight 

 notion of them ; 2nd. The new facts lately discovered about 

 the Che?mstry of Light, and called Spectrum Analysis, and 

 of these I hope you may understand enough to fill you with 

 delight at the beautiful histories they reveal. 



Discovery of the Interference of Light by Young, 1801. — 

 You will remember that Newton and Huyghens had proposed 



