In order to understand how he proceeded, let CED be the convex surface of the one object-glass, and AEB the flat surface of the other. Let them touch at the point E, and let homogeneous _red_ rays fall upon them, as shown in the figure. At the point of contact E, where the plate of air is inconceivably thin, not a single ray of the pencil RE is reflected. The light is wholly transmitted, and, consequently, to an eye above E, there will appear at E a black spot. At _a_, where the plate of air is thicker, the red light _ra_ is reflected in the direction _aa′_, and as the air has the same thickness in a circle round the point E, the eye above E, at _a_, will see next the black spot E a ring of red light. At _m_, where the thickness of the air is a little greater than at _a_, the light _r′m_ is all transmitted as at E, and not a single ray suffers reflection, so that to an eye above E at _m′_ there will be seen without the red ring _a_ a dark ring _m_. In like manner, at greater thicknesses of the plate of air, there is a succession of _red_ and dark rings, diminishing in breadth as shown in the diagram. [Illustration: _Fig. 8._] When the same experiment was repeated in _orange_, _yellow_, _green_, _blue_, _indigo_, and _violet_ light, the very same phenomenon was observed; with this difference only, that the rings were _largest_ in _red_ light, and _smallest_ in _violet_ light, and had intermediate magnitudes in the intermediate colours. If the observer now places his eye below E, so as to see the transmitted rays, he will observe a set of rings as before, but they will have a bright spot in their centre at E, and the luminous rings will now correspond with those which were dark when seen by reflection, as will be readily understood from inspecting the preceding diagram. When the object-glasses are illuminated by _white_ light, the _seven_ systems of rings, formed by all the _seven_ colours which compose white light, will now be seen at once. Had the rings in each colour been all of the same diameter they would all have formed brilliant white rings, separated by dark intervals; but, as they have all different diameters, they will overlap one another, producing rings of various colours by their mixture. These colours, reckoning from the centre E, are as follows:— 1st Order. Black, blue, white, yellow, orange, red. 2d Order. Violet, blue, green, yellow, orange, red. 3d Order. Purple, blue, green, yellow, red, bluish-red. 4th Order. Bluish-green, green, yellowish-green, red. 5th Order. Greenish-blue, red. 6th Order. Greenish-blue, red.