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 composed of green, 

 red and yelloto. 



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 phaenomenon : 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 phaenomenon 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 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 Tiey 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 bvo 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. 



