produced by two Plates of Glass of equal Thickness. 359 



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

 jHnges arc produced in the same manner as it" the ray had passed 

 through AB. 



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

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

 produced as formerly. 



In fig. 4. the refiection from the external surface mn of the 

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

 In this case no fringes are produced. 



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

 ■op of the plate AB are destroyed by a layer of Canada balsam. 

 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 reflection from the external surfaces mn, op, of 

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



In all these cases, the fringes are ob>viously produced by a re- 

 fraction and a reflection in each of the two plates, and the in- 

 terfering fringes are produced by the secondary reflections within 

 the glass plates. 



The fringes, however, which appear xipon the first or bright 

 images, are produced in a different manner from those formed 

 by the light that has been reflected from the plate CD ; for the 

 Sight of which they are composed has suffered two or more re- 

 flections within the plate AB, as shown in fig. 8. and two re- 

 fractions by the plate CD. These refractions are absolutely ne- 

 cessary to the production of the fringes ; for they disappear 

 when the light reaches the eye, without passing through the se- 

 cond ]jlate. Any variation in the distance of the plates, when 

 their inclination and thickness remain the same, ought obviously 

 to produce no change in the appearance of the fringes, as the 

 fits will return in tlie same maimer as before^ 



In order to compare the preceding pha?nomena with the New- 

 tonian Theory &f Fits, I propose to resume the investigation 

 with plates of parallel glass, that differ very considerably in 

 thickness, and that have their surfaces ground as flat and j)n- 

 lished as higldy as possible ; and I have no doubt but that ail 

 the results may be calculated by means of that beautiful theory. 



The fundamental experiment by ivhich I ascertained the pro- 

 duction of coloured fringes by two plates of glass of eciual thick- 

 ness, has been repeated and verifiecl by my friend M. Biot of the 

 Institute of France, and was exhibited at a public meeting of 

 that distinguished body. 



Z4 LXVI. Oh' 



