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LONDON, EDINBURGH, ann DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAT, OF SCIENCE. 
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[SIXTH SERTES.] | ~ 
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JANUARY 1904. \ 

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I. A Simple Explanation of Talbot’s Banils. 
By ArtHur Scuuster, &.RS* | 
4. HESE bands are observed in a spectrum when_half 
the aperture of the pupil is covered with a thin 
plate of mica or glass, provided that the plate be inserted on 
that side on which the blue of the spectrum appears. The 
explanation of these bands which has been given by Airy 
and Stokes involves a rather elaborate mathematical process 
which, though convincing, does not leave the mind com- 
pletely satistied. The essential reason for the want of sym- 
metry which causes the bands to appear only when the plate 
is introduced on one side, ought to be capable of being 
rendered obvious in a more simple manner. This I propose 
to do in the present communication. 
As the bands are seen with “white light,’ a single 
luminous impulse should be sufficient to produce them, and 
as the distribution of intensity in the spectrum is clearly not 
an essential factor in the ease, we may choose the shape and 
duration of the impulse as we like. 
Let an indefinitely short impulse spread out from a distant 
point, and strike a plane grating GG’ normally. This 
grating may be imagined to be made up of a series of narrow 
reflecting parallel strips A;, Ay, &c., separated by intervals 
which refiect no light. A lens SS’ having its tocus at F 
receives the luminous disturbance. The impulsive velocity 
spreading from A, reaches F sooner than that reflected from 
* Communicated by the Author. 
Phil. Mag. 8. 6. Vol. 7. No. 37. Jan. 1904. B 
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