TRANSACTIONS OF THE SECTIONS. 3 
On a New Apparent Polarity of Light. By the AsTRONOMER RoyAL. 
Sir David Brewster had, at the Liverpool and Newcastle Meetings, 
stated a most extraordinary fact respecting the solar spectrum or 
coloured image formed by the agency of a prism, when viewed through 
thin plates of glass or mica. Most members of the Section were aware, 
but to some it might be new, that the light in the solar spectrum could 
be so managed as to be entirely free from all mutual intermixture or 
jumble of different colours. When this was done, suppose this pure 
spectrum to be so turned as that its violet end lay to the right hand and 
the red end towards the left, and suppose the pupil of the eye to be half 
covered by a thin piece of glass or mica—if the piece of glass or mica 
be made to cover that half of the pupil which is towards the violet end 
of the spectrum, numerous parallel bands are seen to cross the spec- 
trum. This fact was long since observed by Mr. Talbot; but the 
extraordinary fact observed by Sir D. Brewster was, that upon turning 
the plate of mica so as to cover the half of the pupil next the red end 
of the spectrum, all the bands completely disappeared. This fact ap- 
peared so inexplicable to Sir David Brewster, that he pronounced it to 
indicate a new and hitherto unobserved polarity of light. From this 
opinion he ventured to dissent, and he should endeavour to explain to 
the Section how complete a solution of the facts was afforded by the 
undulatory theory of light. But before he proceeded, he must premise 
that his own experience of the facts differed from that of Sir D. Brews- 
ter, though so slightly, that the circumstances he deemed material 
might readily be overlooked. He should only say that he had consulted 
a friend respecting these discrepancies long before he was aware of 
their important bearing on the explanation. The peculiarly short- 
sighted character of his eye, was perhaps the occasion of their beco- 
ming so perceptible to him. The tacts, as he observed them, were :— 
1. When a spectrum is viewed out of focus, bands are formed by pla- 
cing a piece of mica, of a proper thickness, so as to cover the half of 
the pupil next the violet end. 2. No bands are formed with any 
thickness of mica if it be placed on the side of the red end. 3. When 
the eye is too distant to see the spectrum distinctly, upon moving the 
mica from the violet end, bands are seen advancing in the same direc- 
tion over the spectrum. 4. When the eye is too near to see the spec- 
trum distinctly, the bands appear to move in the opposite direction. 
5. If the eye be so far off, and the spectrum is consequently seen so 
indistinctly, that the ruddy portions are nearly mingled with the blue, 
upon covering with the mica half the pupil next the violet end, bands 
are seen well-defined, but narrow. 6. If the eye and mica approach 
the position of distinct vision of the spectrum, the bands become 
broader, and near the position of distinct vision sometimes disappear ; 
on approaching still nearer, the bands re-appear and become narrower, 
but he thinks are not seen so distinctly as when the eye is too far off 
(this may, however, depend on the practical difficulty of that part of 
the experiment). 7. Bands which are visible when the mica is on the 
violet side, and invisible when it is on the red side, never occur when 
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