358 On a new Species of coloured Fringes 



When the eye is placed at a considerable distance, either be- 

 fore or behind the glass plates, all the fringes have a very dis- 

 torted appearance, arising probably from the imperfect figure of 

 the reflecting surfaces. 



In order to explain the changes which the light undergoes in 

 its passage through the plates of glass, let AB, CD, Plate VII. 

 fig. I. be a section of two plates at right angles to the common 

 section of their surfaces, and let RS be a ray of light incident 

 nearly in a vertical direction. This rav, after passing through 

 the first plate AB, will suffer a small refraction at P and Q, and 

 emerge in the direction QV parallel to RS. At the point P, in 

 the second j^latc CD, the ray TP will be reflected to a, again 

 reflected to li, and after suffering a refraction at b and c, will 

 emerge in the direction cd, forming with RV an angle equal to 

 twice the inclination of the plates. A. portion of the reflected 

 ray Pa will enter the first plate at a, and ha'.ing suffered re- 

 flection and refraction at |3, the reflected portion ^y will reach 

 the eye at S. The ray Pr//.'C will likewise suffer a reflection at c 

 and at e, and will reach the eve at g. In like manner, a part 

 of the ray PQ will be reflected at Q, and move in the direction 

 Q.rstuv, and another part of it in the direction sii'xyz, and 

 these rays will suffer several other reflections ; but the images 

 which they form will be so faint, that the eye vvill not be capa- 

 ble of perceiving theui. When the oljserver, therefore, looks at 

 a luminous body, in the direction SR, through the glass plates, 

 he will perceive two images, one of which is a bright image, seen 

 by the transmitted light QV, and the other is a faint image, 

 seen principally by the reflected light Vahcd, and composed of 

 several images formed by the pencils cd, uv, efl, 2;^, and eg. 

 The bright image is not crossed by coloured fringes, but the 

 fringes appear distinctly upon the other image; and the light 

 by which these fringes are formed, has suffered two reflections 

 from the exterior surfaces, and two refractions at the interior 

 surfaces of the plates. 



When the ray RS is incident obliquely, so as to produce the 

 coloured fringes, the plane of incidence is parallel to the com- 

 mon section of the plates. In this case, it is difficult to repre- 

 sent in a diagram the progress of the rays, as they are reflected 

 in a plane at right angles to that in which they are refracted. 

 The changes, however, which the light must undergo in the pro- 

 duction of the fringes, may be understood from figs. 2,3,4, 

 5, 6, 7, and 8, where AB and CD are the two plates of glass 

 .inclined at a small angle, and RS a ray of light hicident ob- 

 liquely, in a piano at right angles to the common section of the 

 plate. 



In fig. 2. the plates are so arranged, that the incident ray 



RS 



