382 Mr. Potter on FresneVs Expei'iment 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 

 thi'ough 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 tvas 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- 



