the Superposition of Paragerric Spectra, §c. 



25 



2. They are smaller and more numerous when the grooves 

 are wider or fewer in an inch. 



3. They become smaller and more numerous when the distance 

 of the plates is increased. 



4. They are smaller and more numerous when the angle of 

 incidence is increased. 



5. They become more numerous by increasing the number of 

 reflexions. 



6. They appear like minute black lines upon the colourless 

 image, but when their magnitude is increased, they appear like 

 blue or pink bands on a ground of a different colour, which is 

 generally white or whitish blue. 



These bands were visible on the systems of grooves 312*5, 

 625, 1250, and 2500 in an inch, but not on the systems of 5000 

 or 10,000 in an inch. 



When the spectra had suffered three, four, five, or six re- 

 flexions, 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 : — 



Angular breadth of each . . 7° 50 



Distance of plates 0*22 in 



Angle of incidence 63° 30 



Number of fringes on the disk ) 

 and on the first spectrum. [ 

 Angular breadth of each . . 3° 55 



10 



Distance of plates 0*11 in. 



Distance of circular disk 1155 „ 



Diameter of disk T317 „ 



Angle of incidence .... 63° 30' 

 Angular diameter of disk. 39° 30' 

 Number of fringes on disk [ r 



and on the first spectrum j 



In order to observe the effect produced by varying the angle 

 of incidence, I placed a luminous disk 3/^ inches in diameter* 

 at the distance of 9 feet 6 inches from the grating, and obtained 

 the following results : — 



Angle of 



No. of bands 



Angle of 



No. of bands 



incidence. 



on the disk. 



incidence. 



on the disk. 



70° 



29 



50° 



17 



60° 



21 



40° 



14 



The bands were seen at an incidence of 87£° 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 pre- 

 cise angle of incidence at which they were seen, nor mentioned 

 in my journal whether they were seen with the 625 or the 1250 

 grating :— 



* This disk included part of the spectrum oneach side of the bright image. 



