384? Mr. Potter on FresneVs Exjperiment of Interferences 
usually employed to throw the sun’s light through the win- 
dow-shutter into the darkened room, I have employed an 
equatorial method of mounting, which keeps all the appa- 
ratus in the adjustment it is first placed in, and at the 
same time enables us to follow readily the sun’s daily mo- 
tion, keeping at the same time the room quite dark. 
To obviate all objections which attach to the luminous 
point being formed by a common lens, which has different 
foci for differently coloured rays, 1 have employed a spheri- 
cal mirror, as better even than any achromatic lens; and 
again, as it was suggested to me that an objection might 
be raised if the rays crossed in a real focus, I have used a 
convex mirror, so that they diverge from a virtual focus 
without having crossed : also to enable me to use the lumi- 
nous point the smaller, I have generally used the two mirrors 
slightly inclined, of polished speculum metal, which reflects 
many times the quantity of light which glass reflectors do in 
the position which my apparatus requires. With these pre- 
cautions, I find, when the sun is perfectly unclouded, and 
near the meridian, high above the horizon^ that the central 
band is black. When there are clouds before the sun’s disc, 
however slight, the central band is more difficult to fix upon, 
and generally either white or doubtful. The discrepancies 
which have been to me a puzzle for so many years, I am now 
able to solve, and I announce the following new principle of 
interferences : — 
When light in a state of interference is made to interfere 
again, the residt is of an opposite character to vohat it would 
have been if the light had been in the first instance in the 
ordinary state. 
This proposition, which is in itself reasonable, might have 
been anticipated, and solves all the anomalies. The light 
falling on the small hole in the thin plate of metal, formerly 
mentioned, as used to form the luminous point, is thrown 
into a state of interference by diffraction at the edge of the 
hole, and hence the central band is seen white. Again, the 
sun’s light passing through thin clouds, or through the va- 
pours of the atmosphere when near the horizon, is thrown 
more or less into a state of interference by diffraction at the 
edges of the particles of the vapours, and gives results which 
are either doubtful or with a white centre : for where part 
of the light is in its original state and part in a state of inter- 
ference, the bright and dark centred bands will be super- 
posed, the central band of the one set over the central band 
of the other, and thus produce indistinct phasnomena in which 
the intensity of the one or the other species may prevail. 
