Page:Proceedings of the Royal Society of London Vol 1.djvu/151

Rh but be highly ‘useful, as they may be depended on; the interior surveys of those parts having since proved that no erroneous inter- sections had been made in those operations. -

It consists of six sections, the ﬁrst of which is intended to convey an experimental demonstration of the general law of the interference of light. This demonstration rests on two experiments, the results of which are brought in proof, that fringes of colours, and even the crested fringes described by Grimaldi, are produced by the inter- ference of two portions of light. These results are, that if one of the two edges of a shadow produced by a narrow opake body be intercepted by a screen at a small distance from that body, the op- posite edge will no longer exhibit the fringed appearance which it bad in common with the former edge, when the latter was not inter- cepted.

Under the second head we have a comparison of measures of the intervals of disappearance of light when refracted between two edges of knives, or intercepted by a hair or a thin wire. The experiments, which were partly suggested by some observations of Sir Isaac New- ton, are here collected in tables: .and the author states, as a general inference, that if we thus examine the dimensions of the fringes under different circumstances, we may calculate the differences of the lengths of the paths of the portions of light which have been proved to be concerned in producing those fringes; and we shall ﬁnd, that where the lengths are equal, the light always remains white; but that where either the brightest light, or the light of any given colour, disappears and reappears a ﬁrst, a second, or a third time, the differences of the paths of the two portions are nearly in an arithmetical progression.

In the third section. these principles'are applied to explain the repetition of colours sometimes observed within the common rain- bow, particularly those described in the Philosophical Transactions by Dr. langwith and Mr. Daval. The train of reasoning here ad- duced would lose too much of its evidence by being abridged.

The fourth section is entitled, “ Argumentative inference respect- ing the Nature of Light.” Here we meet with something of a con- troversial nature, in which those who have adopted theories different from that which our author is desirous to establish, are called upon to explain his experiments according to their principles. What ap- pears to him to operate chiefly against the advocates for the projectile hypothesis of light, is, that light moves more slowly in a denser than in a rarer medium, and that hence refraction is not the effect of an attractive force directed to a denser medium.

The fifth section treats of the colours of natural bodies. The nature of the light transmitted by various bodies is here described, but