528 SPECIAL METHODS OF DISCOVERY. 



positions ; lie also found no less than 576 such lines in the 

 solar spectrum. 



It was by means of ingenious experiments and infer- 

 ences that Young, in 1801, discovered the interference of 

 light. MM. Fresnel and Arago also discovered by experi- 

 ment, that two rays of polarised light, if they are derived 

 from the same source and polarised in the same plane, 

 interfere with each other and produce coloured fringes, 

 but not if they are polarised in different planes. Fresnel 

 also, by experiment, discovered that a beam of light pass- 

 ing through the axis of a quartz crystal, consists of two 

 superimposed rays, moving with different velocities. The 

 discovery of polarisation of light by reflection was also 

 the result of experiment, and was made by Malus, a young 

 French officer, in the year 1808. He was looking from 

 his study in the Kue d'Enfer, Paris, lengthwise through 

 a prism of Iceland spar, at the light of the setting sun, 

 reflected from a window of the Luxembourg Palace, which 

 stood opposite ; and turning the prism slowly upon its axis, 

 he observed that whilst in each of two positions of the prism 

 two images of equal intensity of the window appeared, as 

 in the usual double refraction by that crystal, in each of two 

 other positions at right angles to these one of the two 

 images became faint. Meditating upon this singular cir- 

 cumstance, led him to make many more experiments, by 

 which he was led to the discovery that whenever a beam 

 of ordinary white light is reflected from glass at a parti- 

 cular angle, viz., 56 45', the portion reflected possesses to 

 some extent the peculiar characters of one of the beams 

 which has passed through a crystal of Iceland spar. Malus, 

 in the year 1811, applied the term 'polarised* to light 

 possessing such a quality. As I have already remarked, 

 it is the instructed and disciplined mind alone which 

 perceives before it the presence of a new truth; many 



