442 ON A NEW SPECIES OF COLOURED FRINGES 



distorted appearance, arising probably from the imperfect fi- 

 gure 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 XXII. 

 fig. 1. 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 ray 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 plate CD, the ray TP will be reflected to a, 

 again reflected to b, 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 «, and havino- suf- 

 fered reflexion and refraction at /3, the reflected portion /3 y 

 will reach the eye at 6. The ray J* a be will likewise suffer a 

 reflection at c and at e y and will reach the eye at g. In like 

 manner, a part of the ray PQ will be reflected at Q, and move 

 in the direction Q r s t u v 9 and another part of it in the direc- 

 tion s w x y z, and these rays will suffer several other reflex- 

 ions ; but the images which they form will be so faint, that 

 the eye will not be capable of perceiving them. When the 

 observer, 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 P a b c rf, and composed of several images, formed by the 

 pencils c d,uv, e$, z §, and e g. The bright image is not cross- 

 ed by coloured fringes, but the fringes appear distinctly upon 

 the other image ; and the light by which these fringes are 



formed, 



