XXI. On the Diffraction of mi Ohject-glass with a triangular Aperture. 

 By S. Earnshaw, M. A. Of St Johii's College, Cambridge. 



[Read December 12, 1836.] 



The genei'al adoption of Fresnel's theory seems to indicate that the 

 scientific world is convinced that the Newtonian theory is inadequate to 

 the explanation of the phfenomena of Diffraction ; and that the tiieory 

 which ascribes them to reflexion at the edges of the obstacle is equally 

 unsatisfactory ; and hence it is that the phsenomena of this class have 

 been declared by Sir J. Herschel, to form the strongest points of the 

 undulatory theory of light. Professor -iViry also at the end of his paper 

 on " The Diffraction of an Object-glass with a circular Aperture," 

 {Phil. Trans. Vol. v.) has thus stated his opinion of the importance of 

 these piiai'uomena in the present state of science : " The investigation of 

 cases of diffraction similar to that discussed here, appears to me a matter 

 of great interest to those who are occupied with the examination of 

 theories of light." This sentiment was expressed in 1834, and since 

 that time I am not aware that any thing has been done in the com- 

 j)arison of theory with experiment in this class of pha^nomena. It is 

 true, theory may have been applied to certain cases of diffraction, but 

 it does not appear that the persons who have so applied it have ever 

 contemplated more than merely to shew that tlieory gave a result some- 

 tking like the observed phsenomenon. Sucli an inference being wholly 

 useless in the present state of the theory of liglit, there is still need 

 for the minute discussion of particular cases of diffraction, and for tlie 

 impartial comparison of the results of theory and experiment. I have 



