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 aerial, 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 circle, 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 oi'der undergoes little or no change. We must not omit to 

 mention the effiect of refraction, which is to render the transitions from 

 one tint to another more slow ui 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 correspondmg 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 NcAvton'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 phsenomeuon changes. 

 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 tliem the 

 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 mention 

 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. 



