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