relative to the Interference of Light. 437 



interference-experiments, although it is most convenient to 

 have an apparatus like that of a solar microscope for sending 

 the sun's rays horizontally into a dark room, and concentra- 

 ting them by a small lens, whilst the mirror is capable of fol- 

 lowing the motion of the sun, — yet all these conditions are not 

 absolutely essential. A completely darkened room is far from 

 indispensable, and the apparatus may be perhaps more con- 

 veniently fixed in a screen, which may at any moment be 

 placed in the sun's rays in any situation, and the effect ob- 

 served at some feet distance. But the place of such an appa- 

 ratus may be quite sufficiently supplied from materials every- 

 where at hand, by merely adjusting a common plane mirror in 

 a proper position, and receiving the rays either through a small 

 lens, or even a minute aperture in a screen ; the sole essen- 

 tial being an origin of light, which is as nearly as possible an 

 absolute point. Or, again,Ve may use still simpler means; for 

 if we have only a small convex mirror, such as the bulb of a 

 thermometer, a globule of mercury, a polished metallic but- 

 ton, &c. simply placed in the sun's rays, this gives an image 

 of the sun diminished nearly to a point; and if the light from 

 other sources be screened, the diverging beam thus formed 

 will suffice for shov;ing the stripes either with the obtuse prism, 

 or by two reflecting surfaces, not indeed with the same bril- 

 liancy and distinctness, but sufficiently to verify the facts. 



This method is also successfully applicable to the fringes of 

 edges and apertures, and the internal stripes of shadows for- 

 merly called Diffraction. And mentioning these phaenomena, 

 I may observe, by the way, that the simple property on which 

 the explanation of these fringes is founded, viz. the tendency 

 of light to diverge from a new origin whenever an obstacle is 

 presented, — which is a real exception, as far as it goes, to the 

 primary law of the rectilinear propagation of light, — does not 

 appear to me (except, perhaps, in one passage in Sir J. Her- 

 schel's treatise,) to have been placed in that prominent point 

 of view in which it should be stated in the elementary exposi- 

 tion of the nature and propagation of light. There are also 

 some other remarkable facts apparently dependent on it, which 

 I am engaged in investigating. 



One of the most singular experiments connected with the 

 coloured fringes is that in which the centre of the shadow of 

 a small circular disk appears a bright point. This experiment 

 is difficult to perform satisfactorily; since even when such a 

 disk is cut with the utmost care, each of the minute inequali- 

 ties in its edge is magnified, and accompanied with fringes, 

 which mix and cross in such a manner as totally to confuse 

 the whiile appearance. I have succeeded by taking up a 



