of particles or thin layers; the terrestrial, which are grosser and capable of reflecting the green tints; and the aërial, 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 order, 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 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 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 number exhibiting the appearance of the next inferior number. This law prevails until we come to No. 21 : after this the phænomenon 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 them 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[1]. 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.
