by the lefs refrangible rays exceed in magnitude those which are formed by the more refrangible rays, and confequently that in any feries the more D refrangible rays are reflected at lefs thickneffes than those which are lefs refrangible.

If the light be incident obliquely, the rings of E colours dilate and enlarge themselves; whence it follows, that the thickness required to reflect the colours of any feries is different in different obliquities.

Water, applied to the edges of the glaffes, is F attracted between them, and filling all the intercedent space, becomes a thin plate of the fame dimenfions as that which before was conftituted of air. In this cafe the rings become much fainter, but vary not in their fpecies, and are contracted in diameter nearly in the proportion of 7 to 8: confequently the intervals of the glaffes at like circles caused by thefe two mediums, water and air, are as about 3 to 4; that is, nearly as the fines which measure the angles of incidence and refraction, made at a common furface between them. And hence it may be fufpected, that if any other G medium, more or lefs denfe than water, "be compreffed between the two glaffes, their intervals at the rings caufed thereby will be to the intervals at which fimilar rings are caufed by the interjacent air, as the fine which measures the refraction made out of air, into that medium is to the fine of the incidence on the common furface.

These are some of the phenomena of light inci- H

dent

I

dent on mediums which are invironed by mediums of greater density, as air or water compreffed or included between plates of glass. The fame appearances follow, though with fome little variation, when the colorific medium is denfer than that in which it is inclosed.

It is well known that bubbles blown in foapwater exhibit a great variety of colours; but as these colours are commonly too much agitated by the external air to admit of any certain obfervation, it is neceffary that the bubble be covered with a clear glafs; in which fituation the following appearances enfue: the colours emerge from the vertex or top of the bubble, and as it grows thinner by the fubfidence of the water, they dilate into circles or rings parallel to the horizon, which flowly defcend and vanish fucceffively at the bot

tom.

This emergence continues till the water at the vertex becomes too thin to reflect the light, at which time a circular fpot of an intense blackness appears at the top, which flowly dilates fometimes to three quarters of an inch in breadth before the bubble breaks. Reckoning from the black central Spot, the reflected colours are the fame in fucceffion and quality as thofe produced by the aforementioned plate of air, and the appearance of the bubble, if viewed by tranfmitted light, is alfo fimilar to that of the plate of air in like circumftances.

K If the colours be viewed with different obliqui

ties, their place is changed, but not near fo much as in the plate of air.

The end of a small glafs tube or pipe being melt- L ed, by turning the flame of a candle or lamp upon it, by means of a blow-pipe, may be blown into a bubble of an extreme thickness. Such a bubble will exhibit colours of the fame kind as the foregoing, but much more brifk and lively. From M which, and the premised obfervations, it is concluded that a denser medium inclosed by one that is rarer exhibits more lively colours than those which are produced by a rarer medium included in one that is more denfe. It is alfo obfervable, n that the colours produced by reflection from, or tranfmiffion through, denfe fubftances, are lefs fubject to vary by change of the obliquity of the incident light than they are in substances that are

more rare.

By wetting very thin plates of Moscovy glass, o whofe thinnefs occafion the like colours to appear, the colours become more faint and languid, efpecially if wetted on the surface oppofite to the eye; but no variation of their fpecies is produced: fo that the thickness of any plate requifite to produce any colour, feems to depend only on the density of the plate, and not on that of the ambient medium: and hence, if the fufpicion formerly urged be true (283, G), may be known the thickness which thin plates of any tranfparent fubftance have at the place where a given colour in any series is produced. For,

As the fine of the angle of incidence at the common furface

Is to the fine of the angle of refraction out of the given medium into air,

So is the thickness of a plate of air which exhibits the given colour

To the thickness of the given plate.

As lenfes ground to a long radius are neceffary to be used in these experiments, and fuch are not very common, it may be an acceptable piece of information for the learner to know, that their place may be well fupplied by two pieces of plateglafs, or even common glass. If these be previoufly wiped, and then rubbed together, they will foon adhere with a confiderable degree of force, and exhibit various ranges of colours, much broader than thofe obtained by lenfes. One of the most remarkable circumftances attending this method of making the experiment is the facility with which the colours may be removed, or even made to disappear by heats too low to feparate the glasses. It seems most probable, that the operation of heat confifts in augmenting the distance between the furfaces. A touch of the finger immediately causes the irregular rings of colours to contract towards their center in the part touched.

CHAP.

CHAP. VII.

GENERAL INFERENCES RESPECTING THE DISPOSI-
TION TO BE

REFLECTED OR TRANSMITTED,

INTO WHICH THE RAYS OF LIGHT ARE PUT,
BY THE ACTION THAT DEPENDS ON THE
THICKNESS OF THE MEDIUM UPON WHICH
THEY ARE INCIDENT.

THE experiments or obfervations in the laft a chapter being maturely weighed and confidered, indicate the following theorem or general propofition; namely,

Every ray of light in its paffage through any re- & fracting furface is put into a certain tranfient conftitution or state, which in the progress of the ray returns at equal intervals, and disposes the ray, at every return, to be easily transmitted through the next refracting furface, and, between the returns, to be easily reflected by it.

For, by those observations it appears, that one s and the fame fort of rays, at equal angles of incidence on any thin tranfparent plate, is alternately reflected and tranfmitted for many fucceffions; accordingly as the thickness of the plate increases. in arithmetical progreffion of the numbers o, 1, 2, 3, 4, 5, 6, 7, 8, &c. fo that if the first reflection, or that which makes the first or innermost ring of colours, be made at the thicknefs 1, the rays fhall

be