242 



THEORY OF MICROSCOPIC OBSERVATION. 



micrometric measurements, of the further simplification, that 

 cos may be taken as equal to 1, without appreciable error. We 

 then get 



The other points of intersection, corresponding to the second, 

 third, fourth, &c., dark lines, are at precisely the same distance 

 from the one immediately preceding them as the first is from the 



reflecting surface. They all 

 lie in an arc drawn from B 

 through P, which, on account 

 of the minuteness of the 

 distances, is to be regarded 

 as a straight line, and is 

 therefore entirely in the 

 plane of adjustment. The 

 interference lines, if ob- 

 served by homogeneous 

 light, accordingly appear 

 equidistant from one 

 another by the magnitude 



X. They therefore ap- 



ZCi 



proach the nearer, the 

 greater is, i.e., the further 

 the slit is from the reflect- 

 ing-plane, and, when d 

 \ a, have a distance of one 

 wave-length only. 



With these theoretical 

 deductions measurements 

 agree perfectly. In an 

 actual case, for instance, d was equal to 2 '7 5 mm., a = 30 mm., 

 and the distance of the lines in white light 3*1 mic. ; calculation 

 gave the wave-length X = -567 mic., which value corresponds 

 nearly to the mean rays. 



The prismatic colours of the bright interference lines are, in 

 reality, arranged in such a manner, that for the first maximum of 

 intensity the violet is turned to the reflecting surface, and the red 

 away from it. In accordance with the laws of the fusion of colours 



O' 



li 

 FIG. 132. 



