224 



SIR DAVID BREWSTER ON THE BANDS FORMED BY THE 



When the spectra had suffered three, four, five, and six reflexions, the central 

 and other images were covered with the same number of bands, as with two 

 reflexions from the grooved steel; but another series of wider bands was 

 superposed. 



The following results were obtained with grooved surfaces having 1250 

 divisions in an inch :— 



Distance of plates, 

 Distance of circular disc, 

 Diameter of disc, 

 Angle of incidence, 

 Angular diameter of disc, 

 Number of fringes on disc, and 

 on the first spectrum, 



Oil inch. 

 11 55 inches. 



1-317 inch, 

 63° 30' 

 39° 30' 



Angular breadth of each, 

 Distance of plates, 

 Angle of incidence, 

 Number of fringes on the disc, and "J 

 on the first spectrum, . J 



Angular breadth of each, 



7° 50' 



22 inch. 



63° 30' 



10 



3° 55' 



In order to observe the effect produced by varying the angle of incidence, I 

 placed a luminous disc three inches and six -tenths in diameter* at the distance 

 of nine feet six inches from the grating, and obtained the following results : — 



Angle of Incidence. 



70 



60 



No. of Bands on the Disc. 

 29 

 21 



Angle of Incidence. 

 50 

 40 



No. of Bands on the Disc. 



17 

 14 



The bands were seen at an incidence of 874°, when the plates were nearly in 

 contact. 



The following were the colours seen on the two spectra on one side of the 

 colourless image ; but I have not measured the precise angle of incidence at which 

 they were seen, nor mentioned in my journal whether they were seen with the 

 625 or the 1250 grating : — 





First Spectrum. 





Second Spectrum. 



Great Incidences 



White. 



Great Incidences 



Blue. 





Pale Red. 





Bluish. 





Red. 





Less Blue. 





Purple. 





Bluish White 





Blue. 





White. 





Bluish. 





Pale Red. 





Less Blue. 





Red. 



Lesser Incidences 



White. 



Lesser Incidences 



f Purple. 

 i Blue. 



At small angles of incidence, about 42°, the bands become less distinct, and paler 

 in colour, the white becoming yellow and the blue brownish. 



In the systems of grooves, whether on glass or on steel, employed in the pre- 

 ceding experiments, the part of the original surface not removed by the grooves 

 bears a very considerable proportion to the part removed ; but when the grooves 

 occupy a large part of the surface, and the intermediate parts a very small one, 

 a new set of phenomena are produced, which must change in a remarkable 

 manner all the bands of interference. The execution, however, of such systems 



* This disc included part of the spectrum on each side of the bright image. 



