In this way it is only possible for **diffracted** rays to enter the objective.

Consider now the light **diffracted** in a direction many times more oblique than any with which we should be concerned, were the whole aperture uninterrupted, and take first the effect of a single small aperture.

6), and the **diffracted** rays make an angle ¢ (upon the same side), the relative retardation from each element of width (a+d) to the next is (a+d) (sin 9 +sin op); and this is the quantity which is to be equated to mX.

For the alteration of wave-length entails, at the two limits of a **diffracted** wave-front, a relative retardation equal to mndX.

Hence, if a be the width of the **diffracted** beam, and do the angle through which the wave-front is turned, ado = dX, or dispersion = /a ..

The occurrence of sin 4 as a factor in (6) shows that the relative intensities of the primary light and of that **diffracted** in the direction B depend upon the condition of the former as regards polarization.

13, we suppose that a diffracting particle of such fineness is placed at 0 that the **diffracted** pencils of the 1st order make an angle w with the axis; the principal maximum of the Fraunhofer diffraction phenomena lies in F' 1; and the two diffraction maxima of the 1st order in P' and P' 1.

This can be done by cutting off the chief maximum and using only the **diffracted** spectra for producing the image.

The extremely small particles of dust (motes in a sunbeam) in the rays are made perceptible by the **diffracted** light, whilst by ordinary illumination they are invisible.

If 8 and 4' denote the angles with the normal made by the incident and **diffracted** rays, the formula (5) still holds, and, if the deviation be reckoned from the direction of the regularly reflected rays, it is expressed as before by (0+0), and is a minimum when 8 = 0, that is, when the **diffracted** rays return upon the course of the incident rays.