Asttonomkal and Nautical Collection^. 131 



and on the other side with the following semiundulation, 

 and then be separated from this again, and at a distance 

 twice as great as the first, must coincide with the second 

 semiundulation before and behind it, of which the actions 

 will coincide with its own : whence there will arise, on 

 the surface of this undulation, a series of lines, at equal 

 distances from each other, in which the motion is destroyed 

 and doubled alternately by the action of the second series. 

 Thus if we receive this luminous undulation on a white card, 

 we shall observe on it a series of dark and bright stripes, if 

 the light employed is homogeneous ; or coloured fringes of 

 different tints, if we employ white light for the experiment. >' 



R A C B _1SL. 



INI JV r 



\ \ //xx 



• 'This will be more easily understood by the inspection of a 

 figure, which represents a section of the two mirrors and of 

 the reflected undulations, formed by a plane drawn from the' 

 luminous point perpendicularly to thfe mirrors represented 

 by DE and DF. The luminous point is supposed to be 

 S, and A and B are the geometrical positions of its two' 

 images, which are determined by the perpendiculars SA and 

 SB falling from Son the mirrors, taking in them PA = SP. 



K 2 



