436 ON A NEW SPECIES OF COLOURED FRINGES 



was a mistake, and that the coloured fringes constituted a 

 new class of phenomena, having a different origin from those 

 of thick plates, though explicable by the beautiful theory of Fits 

 of easy reflexion and transmission, by which Newton was ena- 

 bled to explain all the phenomena of the colours of thick and 

 thin plates. 



In order to observe the phenomenon to the greatest advan- 

 tage, let the light of a circular image subtending an angle of 

 1° or 2°, be incident perpendicularly, or nearly so, upon two 

 plates of parallel glass, placed at the distance of one-tenth of 

 an inch, and let one of the plates be gently inclined to the 

 other, till one or more of the reflected images be distinctly se- 

 parated from the bright image formed by transmitted light, 

 and received upon the. eye, placed behind the plates. Under 

 these circumstances, the reflected image will be crossed with 

 about fifteen or sixteen beautiful parallel fringes : The three 

 central fringes consist of blackish and whitish stripes, and the 

 exterior ones of brilliant stripes of red and green light ; and the 

 central fringes have the same appearance in relation to the 

 external fringes, as the internal have to the external rings, form- 

 ed either by thin plates, or by the action of topaz upon polarised 

 lio-ht. If the two plates of glass are turned round in a plane at 

 right angles to the incident ray, the reflected images will move 

 round the bright image, and the parallel fringes will always 

 preserve a direction at right angles to a line joining the centres 

 of the bright and reflected images. Hence it follows, that the 

 direction of the fringes is always parallel to the common section 

 of the four reflecting surfaces, 'which exercise an action upon the 

 incident light. 



The position of the plates remaining as before, let the incli- 

 nation of the plates, or what is the same thing, the distance of 



the 



