. La lumière, un manuel pour les étudiants qui ont eu une année de la physique. Ni est l'échec de la réflexion régulière une opposition,quand on sait que la distance entre les atomes d'une solidbody est quelque chose de l'ordre de 10s à 107 em.,que l'onde de la lumière visible est de l'ordre de5 X 105, et que si les rayons x ont une longueur d'onde c'est que probablyless 108. Une longue chaîne d'atomes pourraient se situer dans un singlewavelength de lumière jaune, tandis que d'autre part un certain nombre de longueurs d'onde des rayons x pourrait se situer entre deux atomes voisins.Une surface que l'on peut considérer comme poli finement pour

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. Light, a textbook for students who have had one year of physics. Neither is the failure of regular reflection an objection, when we know that the distance between the atoms of a solidbody is something of the general order of 10s to 107 em., that the wavelength of visible light is in the neighborhood of5 X 105, and that if X-rays have a wavelength it is probablyless than 10~8. A long string of atoms could lie within a singlewavelength of yellow light, while on the other hand a numberof wavelengths of X-rays might lie between two adjacent atoms.A surface that we may regard as finely polished for visible REFLECTION OF X-RAYS 265 light, would therefore be an exceedingly rough structure forthe very short waves. In the discussion of reflection in chapterIV, it was stated that each point in the straight line MN offigure 23 representing the reflecting surface becomes the centerfor a secondary wavelet, but it would no doubt have been morein accordance with facts if it had been said that each atom ofthe surface became such a center. Figure 132 represents what. B -I A i— Jf Figure 132 a reflecting surface might look like if it were magnified tillindividual atoms became clearly perceptible. The continuousstraight line MN of figure 23 has become a somewhat irregularrow of atoms forming the top layer of a body composed ofmany such atoms. When the wavefront WF strikes any oneatom it becomes a center for a secondary wavelet. 1, 2, 3, 4and 5 are the respective wavelets from the atoms a, b, e, d, ande. Since these atoms are not on a straight line, the secondarywavelets do not lie tangent to a straight line, but if the wave-length of the light is as long as AB the amount by which eachwavelet falls off a common tangent such as the dotted line isa ver}? small fraction of a wavelength, and therefore the second-ary waves are nearly in phase along this line. Therefore, forvisible light we are justified in treating the polished surfaceof a solid as if it were a continuous surface. But if

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