102 M.NOBILI ON COLOURS, AND ON A NEW CHROMATIC SCALE 
of particles or thin layers ; the terrestrial; which are grosser and capa» 
ble of reflecting the green tints ; and the aérial, which are more subtile 
and capable of reflecting the azure tints. 
Laws of Varying Colours. 
Newton had observed that the colours of the rings changed their 
position as the angle of incidence, under which they were viewed, was 
changed. In certain rings a certain colour viewed at an incidence 
nearly perpendicular appears to form a given eircle, but expands and 
forms a larger circle if viewed obliquely. These changes are much 
more perceptible in the outer than the inner rings. An obliquity of 40°, 
for instance, is sufficient to change the tone of a colour of the fourth or- 
der, though at the same angle of incidence a colour of the first or the 
second order undergoes little or no change. We must not omit to 
mention the effect of refraction, which is to render the transitions from 
one tint to another more slow in proportion to the greater density of the 
substance which forms the thin layer. This law may be included in 
the first, because the rings produced by dense layers are interior in 
reference to the corresponding rings produced by layers of inferior 
density, and the exterior rings are the more liable to change. 
The colours of the scale are produced by thin plates, and are subject 
to the same laws as those of Newton’s rings. It seems to me, however, 
that in respect to the law of the changing colours there is an anomaly 
that has not yet been mentioned. The higher tints comprised between 
the red (No. 44) and the yellow (No.21) conform to the general law. 
If we view these tints at a certain inclination, we see No. 44 change to 
No. 43, No.43 to No. 42, and so on in succession; each superior num- 
ber exhibiting the appearance of the next inferior number. This law 
prevails until we come to No. 21: after this the phenomenon chahges. 
The beautiful yellows 19 and 20 become azure-green; the brighter 
yellows 18 and 17 are changed to red; the azures 16 and 15 become 
yellowish ; the blues 14 and 13 suffer no change, and with them tlie 
anomaly ends, for the general law prevails again from No. 12 to No. 1 
inclusive. : 
This difference has not been indicated until now, and, as I mentidn 
it for the first time, I deem it necessary to state that it escapes the eye 
when we endeavour to observe it in Newton’s rings, in consequence 
perhaps of their being so limited in extent*. The anomaly prevails in 
the central part of the second ring, where the thin plate reflects a great 
quantity of white light, and this part is the brightest of our scale. I 
remark this circumstance; in order that it may receive due attention 
from those who would thoroughly investigate this point. In such an 
investigation it will probably be necessary to take into account the 
* See additional Note at the end. 
