The application of the theory of alternate fits of reflection and transmission to explain the colours of thin plates is very simple. When the light falls upon the first surface AB, Fig. 8 of the plate of air between AB and CED, the rays that are in a fit of reflection are reflected, and those that are in a fit of transmission are transmitted. Let us call F the length of a fit, or the distance through which the particle of light moves while it passes from the state of being in a fit of reflection to the state of being in a fit of transmission. Now, as all the particles of light transmitted through AB were in a state of easy transmission when they entered AB, it is obvious, that, if the plate of air at E is so thin as to be less than one-half of F, the particles of light will still be in their disposition to be transmitted, and consequently the light will be all transmitted, and none reflected at the curve surface at E. When the plate becomes thicker towards _a_, so that its thickness exceeds half of F, the light will not reach the surface CE till it has come under its fit of reflection, and consequently at _a_ the light will be all reflected, and none transmitted. As the thickness increases towards _m_, the light will have come under its fit of transmission, and so on, the light being reflected at _a_, _l_, and transmitted at E, _m_. This will perhaps be still more easily understood from _fig. 9_, where we may suppose AEC to be a thin wedge of glass or any other transparent body. When light is incident on the first surface AE, all the particles of it that are in a fit of easy reflection will be reflected, and all those in a fit of easy transmission will be transmitted. As the fits of transmission all commence at AE, let the first fit of transmission end when the particles of light have reached _ab_, and the second when they have reached _ef_; and let the fits of reflection commence at _cd_ and _gh_. Then, as the fit of transmission continues from AE to _ab_, all the light that falls upon the portion _m_E of the second surface will be transmitted and none reflected, so that to an eye above E the space _m_E will appear black. As the fit of reflection commences at _ab_, and continues to _cd_, all the light which falls upon the portion _nm_ will be reflected, and none transmitted; and so on, the light being transmitted at _m_E and _pn_, and reflected at _nm_ and _qp_. Hence to an eye above E the wedge-shaped film of which AEC is a section will be covered with parallel bands or fringes of light separated by dark fringes of the same breadth, and they will be all parallel to the thin edge of the plate, a dark fringe corresponding to the thinnest edge. To an eye placed below CE, similar fringes will be seen, but the one corresponding to the thinnest edge _m_E will be luminous. [Illustration: _Fig. 9._]