120 M. NOBILI ON COLOURS, AND ON A NEW CHROMATIC SCALE 
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 panne of green, 
red and yellow. 
When the rings are smaller, as they usually are when ofitained 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 phenomenon: 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 di- 
mensions 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 phenomenon connects itself still more with the 
law of refraction. In my opinion there are but few facts that can puta 
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 sur- 
face the coloured rings as much enlarged as is requisite for the conve- 
nient 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 re- 
spects ; they will serve, for instance, as a key to the chromatic scale, 
which is in reality no more than the development of the rings them- 
selves ; and this development is indispensable when we would judge of a 
colour. In the coloured rings, however large they may be, there is al- 
ways 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 re- 
medy 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 enu- 
merate here. 
Reggio, June 29, 1830. 
