as an expevimentum crucis as to the nature of Light. 381 
light; and it must be confessed that the latter theory was 
greatly advanced in probability by the demonstration of the 
former. Certain circumstances, however, such as the colour 
and arrangement of the bands, are required to be examined 
before we can consider the primary theory to be confirmed in 
the same extent as its subordinate theory. For instance, ac- 
cording to the fundamental property of wave interferences 
the central band must be white ; but if it should be found in 
the experiment that the central band is black, this discre- 
pancy, whilst it would not weaken the demonstration of the 
theory of interference, would yet be a fatal objection to the 
theory of undulations. 
Now if two equal series of circular or spherical waves 
which have the same direction, or nearly so, arrive at any 
points in the transmitting fluid, in such a manner that the 
like parts of the waves arrive at the same instant, then their 
conjoint effect will be to produce a resultant wave stronger 
than either of the component waves. If, however, the two 
series are in a state of complete discordance, so that one series 
would produce an effect equal and opposite to that which would 
be produced by the other series, they would counteract each 
other, that is, no resultant wave would be produced. Two 
series of waves, which are respectively of equal diameters, and 
similar, at every instant may be denominated simultaneous; 
and if we draw a line bisecting perpendicularly that joining 
their origins, the waves of each series will meet in the same 
state everywhere along this line, since every point in it is 
equally distant from the two origins. When these waves are 
very distant from the origins compared with the distance of 
the origins from each other, they will have very nearly the 
same directions, and we shall have resultant waves of greater 
strength than the component waves. This is a conclusion de- 
pending only on the fundamental properties of waves, and 
does not involve any review^ of the various hypotheses which 
are held, as to the nature of the vibrations of the particles of 
the fluid through which the waves are propagated. 
7fhe experiment with the two mirrors, before mentioned, 
produces the case we have just discussed ; for the single pencil 
diverging from the original luminous point, is made into two 
pencils after reflection by the mirrors; and these two, if light 
consist of waves, are composed of waves both equal and si- 
multaneous. We have here then an experiment on which to 
test the wave theory of light, and the experiment itself show^s 
us which is the band corresponding to the line bisecting per- 
pendicularly that which joins the luminous images of the 
point. For if the luminous point be formed of white light, 
