D O C . 2 7 8 I N T E R F E R E N C E O F C A N A L R AY L I G H T 2 9 7 the frequency , where the diffraction at the slit must be taken into ac- count. The slit must be wide but not infinitely wide compared to the wavelength λ. According to the theory of diffraction, the intensity of the light diffracted from the incident direction, characterized by α, in the direction of the positive X-axis, is pro- portional to the quantity . In this expression, λ can be substituted by without causing any error of signif- icance. However, it should be taken into consideration that the difference between λ and λ0 has a substantial influence on the outcome of the interference process with the path difference d. Monochromatic radiation passing vertically through the in- terference apparatus will have an intensity behind the apparatus that is proportional to the quantity . In this expression, the dependence of the value of λ on α is essential. The intensity of the radiation reaching , with respect to its dependence on d, is, according to ∞what has been said, defined by the integral where . The integration gives, apart from an insignificant constant multiplier, the value , or, the value 1, depending on whether or . ν0© 1 v c -- α¹ + § · πb λ ------ α· © ¹ sin§ πb λ ------ α¹· © § ----------------------- - 2 λ0 cos2§ π d· λ¹ -- - © [p. 339] x ∞ = +∞ ´ µ µ ¶ –∞ sin2© πb λ0 ------ α¹ § · πb λ0 ------α¹· © § 2 ------------------------- -cos2§ π--· d λ¹ - © dα λ λ0© 1 v c -- α· – ¹ § = 1 1 dv bc¹ ----· - – © § π-----¹ d λ0 § · cos© + d 2bc v -------- - d 2bc- v --------