THE 

 LONDON and EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE, 



[THIRD SERIES.] 



AUGUST 1840. 



XIII. On the Theory of the dark Bands formed in the Spectrum 

 from partial Interception by transparent Plates, By the 

 licv. Baden Powell, M.A., F.R.S., F.G.S., F.R.Ast.S., 



Savilian Professor of Geometry in the University of Oxford*. 



(1 .) f T , HE phenomenon of peculiar dark bands crossing the 

 •*■ prismatic spectrum, when half the pupil of the eye 

 (looking through the prism) is covered by a thin plate of any 

 transparent substance, the edge being turned from the violet 

 towards the red end of the spectrum, was first described by 

 Mr. Fox Talbot in 1837 (Lond. and Edinb. Phil. Mag. and 

 Journal of Science, vol. x. p. 364.), who showed that these 

 bands are due to the interference of the two halves of each 

 primary pencil, one of which is retarded by the plate. 



(2.) Sir David Brewster has given various new modifications 

 of these experiments (British Association Reports, vol.. vii. 

 Trans, of Sections, p. 13.), the most material of which tend to 

 show that the effect is fully produced only when the plate is 

 in the position just described, and diminishes and disappears 

 as it revolves in its own plane ; the same observation being 

 also extended to the case of the spectra formed by inter- 

 ference from grooved surfaces, or gratings. 



(3.) The explanation given by Mr. F. Talbot accounts for 

 the production of the bands simply, but assigns no reason why 

 the interception must take place on one side more than the 

 other. That it does so, is considered by Sir David Brewster 

 as indicating an entirely nevo properly of light; having refer- 

 ence to the different sides of the pencil related to their posi- 

 tion of greater or less refrangibility, and which he has not 

 inexpressively termed a peculiar " polarity." 

 * Communicated by the Author. 



Phil. Mag. S. 3. Vol. 17. No. 108. Aug. 1840. G 



