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HISTORY OF SCIENCE. 



centre of the darkest part. A reason for the greater width of the first 

 bands will be suggested by an inspection of the diagram, Fig. 208. 

 With the same admirable clearness that characterizes his account of 

 these experiments and deductions, Fresnel in the same paper applies 

 the wave theory to explain how it happens that a small hole or the 

 focus of a lens becomes practically a centre of luminous undulations. 

 He even traces the undulations through the lens by which the light 

 and dark fringes are viewed, and follows the waves in their progress 

 into the eyeball itself, until they impress the retina. 



In order that the reader may understand the reasons which led 

 Fresnel to make transverse vibrations a part of the undulatory theory 

 of light, it will be necessary to here describe particularly some of the 

 phenomena of polarized light. There is a crystallized mineral called 



tourmaline found in various localities. Chemically it consists essen- 

 tially of silica and alumina, together with small quantities of lime and 

 other bases, including iron and manganese. Tourmalines occur of 

 various colours, and in some kinds the polarizing property is more 

 effective than in others. The mineral was first discovered in Ceylon 

 in the sixteenth century, and has since been found in many other 

 places. Fig. 210 shows at A and B the prismatic shapes of the tour- 

 maline crystals, c represents such a crystal of tourmaline cut by 

 means of a lapidary's wheel into slices parallel to the axis of the prism. 

 The two inner slices, which may have each a thickness of about 2 Vth 

 of an inch, form, when polished, a convenient means of exhibiting the 

 simplest phenomena of polarized light. Indeed, the tourmaline plates 

 were for a long time the only means at the command of experimen- 

 talists. If one of these plates be held vertically between the eye and 

 a source of ordinary light the flame of a candle, for instance the 

 light will be distinctly seen through it, tinged however with the colour 

 of the tourmaline, which is usually greenish. In this respect it will 

 make no difference whether the slice be held with the axis of the crystal 

 upright, horizontal, or inclined, the light will be seen equally well. 

 Let us suppose, however, that this slice is fixed with the axis vertical 

 between the spectator's eye and the light. Then let a second slice of 

 tourmaline, either from the same or from another crystal, be held 



