PRODUCED BY TWO PLATES OF GLASS OF EQUAL THICKNESS. 443 



formed, has suffered two reflexions 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 

 common section of the plates. In this case, it is difficult to 

 represent in a diagram the progress of the rays, as they are re- 

 flected in a plane at right angles to that in which they are re- 

 fracted. The changes, however, which the light must under- 

 go in the production 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 inci- 

 dent obliquely, in a plane at right angles to the common sec- 

 tion of the plate. 



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

 RS does not pass through the first plate AB. In this case, 

 the fringes are produced in the same manner as if the ray had 

 passed through AB. 



In fig. 3. the rays reflected from the plate AD do not pass 

 through the second plate CD. In this case, the fringes, are 

 produced as formerly. 



In fig. 4. the reflection from the external surface m n of the 

 plate AB, is destroyed by a layer of indurated Canada balsam. 

 Jn this case no fringes are produced. 



In fig. 5. the refraction and reflection at the interior surface 

 op of the plate AB, is destroyed by a layer of Canada bal- 

 sam. In this case no fringes are produced. 



In fig. 6. the refraction of the interior surface of the plate 

 CD is destroyed by a layer of Canada balsam, and in this case 

 no fringes are produced. 



In fig. 7. the reflexion from the external surfaces m n, op, 

 of the two plates is destroyed, and no fringes are produced. 



In 



