382 Mr. Potter on FresneVs Experiment of Interferences 
the various colours having different intervals for their lumini- 
ferous surfaces or different lengths of waves in the undulatory 
theory of light, the bands will be of different breadths for dif- 
ferent colours, but will have the one on the bisecting line 
above named bright for every colour, and which will be con- 
sequently white, when the luminous point is formed with 
white light ; but on each side of it, the superposition of bands 
of different breadths corresponding to different colours, will 
cause the compound bands to be coloured, at first on their 
edges only; afterwards the colours will become more and 
more spread, and the bands at the same time more confused, 
as the distance from the central band becomes greater, until 
they are at length gradually lost in a light uniform to the eye. 
The bands on each side and near to the central one will have 
their inner edges violet and their outer edges red, so that the 
arrangement of colours will be symmetrical on each side of 
the central white band. The central band is thus pointed 
out in the experiment by the arrangement of the colours. 
In my early trial of this experiment, I happened to have a 
clear sky and unclouded sun, which afforded a result causing 
me to hesitate before I accepted the undulatory theory as 
true. I have seen the experiment on days in which there 
were thin clouds in the atmosphere, and once when the sun 
was near the horizon, such that it would have led me to a 
different conclusion. 
In my first experiments I was surprised to find the colours 
symmetrical on each side of a dark band, and not a bright 
one. Every precaution was used, such as keeping the bands 
clear of the diffracted fringes formed by the edges of the mir- 
rors, making the distance between the luminous images so 
small that the bands were very large, and therefore that any 
prismatic effect produced by slight error of looking centrically 
through the eye-lens did not produce a sensible effect in the 
arrangement of the colours; also care was taken that the di- 
rect rays (as the term direct is used in optics in contradistinc- 
tion to oblique) passing through the lens forming the lumi- 
nous point were those which fell on the two mirrors. Still the 
central band was a dark one and not a bright one. I ob- 
tained the assistance of friends accustomed to accurate obser- 
vation, to examine the appearances, and they came to the 
same conclusion. I was thus at a loss to conceive how the 
advocates of the undulatory theory could state that the cen- 
tral hand mis always a white one. Some time afterwards, 
however, I obtained a different result; for experimenting one 
fine evening when the sun was near the horizon, I saw the 
middle band clearly white, and the colours accurately symme- 
