My rings can easily be so enlarged that the intruded ring may occupy a space two or three lines in breadth. The tints composing it will then be seen very distinctly, and will correspond exactly with those which are seen in detail on the plates 20, 19, 18, 17, 16 and 15; with this difference only, that, in place of these tints, a ring will be seen composed of green, red and yellow.
When the rings are smaller, as they usually are when obtained under the platina point, the intruded ring appears in the same place, and the observation, though made under circumstances less favourable, is equally decisive.
Newton's rings give no idea of this phænomenon: they vanish from the eye of the observer before the last degrees of obliquity are attained, and are consequently unavailable in an observation for which these great inclinations are an indispensable condition. The smallness of the dimensions of the rings cannot cause the observation to fail, whenever it can be made on my rings whether large or small.
I cover a portion of my rings with a layer of alcohol, oil, or water, &c., and when the observation is made at the before-mentioned inclination of from 70° to 80°, the intruded ring appears only where the humid layer is wanting. Thus the phænomenon connects itself still more with the law of refraction. In my opinion there are but few facts that can put a theory so severely to the test as this, and the theory which can completely explain it will have every claim to credit.
I shall always add to my chromatic scales a plate exhibiting on its surface the coloured rings as much enlarged as is requisite for the convenient study of the properties of the intruded ring. This I feel the more inclined to do, as these large rings are likely to be useful in other respects; they will serve, for instance, as a key to the chromatic scale, which is in reality no more than the development of the rings themselves; and this development is indispensable when we would judge of a colour. In the coloured rings, however large they may be, there is always found between every two tints a third into which they melt: its tone and the feeling which it produces are always confounded with those of the contiguous tints. For this inconvenience there is no remedy but to isolate the tints, so that the eye may be fixed on each of them without receiving at the same time any sensation from the others. The chromatic scale affords this advantage in its detached plates, not to mention the other advantages which in the course of this Memoir it has been proved to possess, and which it is therefore unnecessary to enumerate here.
Reggio, June 29, 1830.
