S54 On a new Species of coloured Fringes 



a different origin from those of thick plates, though explicable' 

 by the beautiful theory of fits of easy rellection and transmission' 

 by which Newton was enabled to explain all the phsenoniena of 

 the colours of thick and thin plates. 



In order to observe the phffiuonienon to the greatest advan- 

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

 1"^ or 2^ be incident perpendicularly, »r 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,^ 

 til! one or more of the reflected images be distinctly separated- 

 from the bright image formed by transmitted light,, and received- 

 upon the eve placed behind the plates. Under these circum- 

 stances the reflected image will be crossed with a!)Out fifteen or 

 sixteen beautiful parallel fringes : the three central fringes consist 

 of blackish and whitish stripes, and the exterior ones ofbrilHant 

 stripes of red and green light ; and the central fringes have the 

 same appearance in relation to the external fringes, as the in- 

 ternal have to the external rings, formed either by thin plates,^^ 

 or by the action of topaz upon polarized light. If the two plates- 

 of glass are turned round in a plane at right angles to the inci- 

 dent ray, the reflected images will move round the bright image, 

 and the jiarallel fringes will ahvays preser\'e 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, luhich exercise an action 7ipon 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 bright and the reflected image,, be varied by a gentle motion- 

 of one of the plates, the coloured fring-es will be found to in- 

 crease in breadth as the inclination of the plates is diminished,- 

 and to diminish as the inclination of the plates is increased. 



In order to determine the law according to which the mag- 

 nitude of the fringes varies, I employed two plates of parallel glass 

 ^'-^y^^dths of an inch thick, and obtained the following measures 

 for the fringes which crossed the image that had suffered two 

 reflections between the plates. The pencil of light was incident 

 nearly in a vertical direction upon the first plate. 



Incliiiation of the Plates. Angular brerxfith of each Fringe. 



1° 1 1' 26' 50'' 



2 20- 13 3 



5 36 5 41 



Now since 5° ?6' : 26' 50" : : 1° 1 1' : 5' 40", and since 



5 36 : 13 3 : : 2 20 : 5 27, it follows, that 

 the breadth of the fringes is inversely as the inclination of the 

 plates, 



OwiMg. 



