98 HISTORY OF OPTICS. 



cal optician of Munich. lie made a great number of experiments on 

 the shadows produced by small holes, and groups of small holes, very 

 near each other. These were published 8 in his New Modifications of 

 Lir/ht, in 1823. The greater part of this Memoir is employed in 

 tracing the laws of phenomena of the extremely complex and splendid 

 appearances which he obtained ; but at the conclusion he observes. 

 " It is remarkable that the laws of the reciprocal influence and of the 

 diffraction of the rays, can be deduced from the principles of the 

 undulatory theory : knowing the conditions, we may, by means of an 

 extremely simple equation, determine the extent of a luminous wave for 

 each of the different colors ; and in every case, the calculation corres- 

 ponds with observation." This mention of "an extremely simple 

 equation," appears to imply that he employed only Young's and 

 Fresnel's earlier mode of calculating interferences, by considering two 

 portions of light, and not the method of integration. Both from the 

 late period at which they were published, and from the absence of 

 mathematical details, Fraunhofer's labors had not any strong influence 

 on the establishment of the undulatory theory ; although they are 

 excellent verifications of it, both from the goodness of the observa- 

 tions, and the complexity and beauty of the phenomena. 



We have now to consider the progress of the undulatory theory in 

 another of its departments, according to the division already stated. 



Sect. 3. Explanation of Double Refraction by the Undulatory 



Theory. 



WE have traced the history of the undulatory theory applied to 

 diffraction, into the period when Young came to have Fresnel for his 

 fellow-laborer. But in the mean time, Young had considered the 

 theory in its reference to other phenomena, and especially to those of 

 double refraction. 



In this case, indeed, Huyghens's explanation of the facts of Iceland 

 spar, by means of spheroidal undulations, was so complete, and had 

 been so fully confirmed by the measurements of Haiiy and Wollaston, 

 that little remained to be done, except to connect the Iluyghenian 

 hypothesis with the mechanical views belonging to the theory, and to 

 extend his law to other cases. The former part of this task Young 

 sxecuted, by remarking that we may conceive the elasticity of the 



' In Schumacher's Astronomische Abhandhmc/en,iu.'Frenc'k ; earlier in German 



