2 Rev. B. Powell on Experime7its relative to 



Dutour found the effect vary with the distance of the object 

 from the orii^in of light, and ascribed it to some cause of un- 

 equal divergence in the ray passing near the edge at different 

 distances from the origin : he attributed the diffraction to an 

 atmosphere about the body less refractive than air. Biot sug- 

 gested a diminishing reflective power. Dr. Young examined 

 at large the hypothesis of an inflecting atmosj)here, and men- 

 tions that Grimaldi and Hooke supposed the inflexion due to 

 " the natural tendency of light to diverge." (Lect. ii. p. 629.) 

 Biot examined particularly the nature of the action ascribed 

 to the edge, in connection with the nature of the surrounding 

 medium, by inclosing the edge or aperture in a medium of 

 water, using glass edges immersed in a liquid of very nearly 

 the same refractive power, &c. ; from whence he concluded that 

 the edge must be viewed simply as a boundary. [Traite de 

 Phys. iv. 743.) More lately M. Haldat has shown that the 

 action of the edge is not affected by heat, varied from ordinary- 

 temperatures to a white heat, nor by magnetism, galvanism, 

 &c. : hence he denies the existence of any peculiar atmosphere. 

 (Quar. Journal of Science, N.S. No. xii. p. 39'^.) 



The suggestion of Dr. Young (Lect. i. p. 467.), that the 

 light reflected from the edge occasioned the fringe by its in- 

 terference with that which passed at a little distance, seems 

 rendered superfluous even by his own view of undulations di- 

 verging from a new centre at the edge (Lect. ii. p. 620); and 

 this last principle simply has more recently been shown, by 

 Fresnel, to supply an explanation, in which the coincidence be- 

 tween the mathematical theory and the results of the nicest 

 measurement possesses a degree of precision irresistibly com- 

 manding our assent. (Herschel on Light, p. 718.) Very re- 

 cently a theory has been proposed in a paper by Mr. Barton, 

 read before the Royal Society, which seems to unite as it 

 were the principles of emission and undulation: it remains to 

 be ascertained, whether, supposing it to explain pheenomena 

 of this class, it will apply equally well to others, especially to 

 the interferences of polarized light*. 



(3.) The earlier experimenters on the subject inferred that 

 the edge of a body exercised a peadiar action on light: and 

 their hypotheses were no more than attempts to refer this to 

 some mode of physical action, such as attraction or repulsion, 

 already recognised in other phaenomena. The later explana- 

 tions have tended to connect this particular instance of an 

 effect of light with numerous others not at first sight appearing 

 to bear any relation to it ; they have referred the pha;nomena 



• An abstract of Mr. Barton's paper will be found in Phil. Mag. and 

 Annals, N.S. vol. x. p. ."JOO.— Edit. 



to 



