4 Rev. B. Powell on Experiments relative to 



placing a small slip of paper under its outer edge. They 

 should be at the distance of several feet from the origin of 

 light, and viewed by an eye lens at about six inches di- 

 stance. 



This method is in fact the easiest and most satisfactory way 

 of showing the experiment, being absolutely free from com- 

 plication by any adventitious causes, (some of which occur 

 in the method of refraction by means of the obtuse prism,) and 

 requiring no apparatus but what is everywhere to be pro- 

 cured. 



The origin of light is here supposed to be the focus of a 

 small lens, by which are concentrated the sun's rays reflected 

 into a dark room in a convenient direction, by means of a 

 plane mirror outside the window-shutter. It would be highly 

 desirable if any experimenter could succeed in rendering 

 lamp or gas light available for the purpose of these experi- 

 ments. 



(8.) The general theory of the fundamental experiment on 

 interference by means of the obtuse prism or two reflectors, 

 has not been given in any treatise in a simple and satisfac- 

 tory form, so as to point out the relation between the actual 

 breadths of the dark and bright intervals observed and the 

 lengths of undulations, with the exception of the excellent 

 Syllabus of Professor Airy. The result of the investigation 

 there given by means of a geometrical construction for the 

 obtuse prism, might however be obtained in a simpler manner 

 for the general case of two rays interfering at a small angle by 

 whatever means. For if we merely consider two very small 

 portions of systems of spherical waves interfering, (the con- 

 ception of which may be assisted by the annexed 'diagram,) 



we shall easily see that if c be the interval between two suc- 

 cessive bright stripes, A the length of an undulation, and 2 $ 



the 



