1893.] on Interference Bands and their Applications. 73 



so combined that the exterior surfaces are parallel to one another, but 

 inclined to the interior operative surfaces. In this arrangement the 

 false light is thrown somewhat to one side, and can be stopped by 

 a screen suitably held at the place where the image of the electric arc 

 is formed. 



The formation of colour and the ultimate disappearance of the 

 bands as the interval between the surfaces increases, depends upon 

 the mixed character of white light. For each colour the bands are 

 upon a scale proportional to the wave-length for that colour. If we 

 wish to observe the bands when the interval is considerable — bands 

 of high interference as they are called — the most natural course is to 

 employ approximately homogeneous light, such as that afforded by a 

 soda flame. Unfortunately, this light is hardly bright enough for 

 projection upon a large scale. 



A partial escape from this difficulty is afforded by Newton's ob- 

 servations as to what occurs when a ring system is regarded through 

 a prism. In this case the bands upon one side may become approxi- 

 mately achromatic, and are thus visible to a tolerably high order, in 

 spite of the whiteness of the light. Under these circumstances there 

 is, of course, no difficulty in obtaining sufficient illumination ; and 

 bands formed in this way were projected upon the screen.* 



The bands seen when light from a soda flame falls upon nearly 

 parallel surfaces have often been employed as a test of flatness. Two 

 flat surfaces can be made to fit, and then the bands are few and broad, 

 if not entirely absent ; and, however the surfaces may be presented 

 to one another, the bands should be straight, parallel, and equi- 

 distant. If this condition be violated, one or other of the surfaces 

 deviates from flatness. In Fig. 2, A and B represents the glasses to 

 be tested, and C is a lens of 2 or 3 feet focal length. Rays diverging 

 from a soda flame at E are rendered parallel by the lens, and after 

 reflection from the surfaces are recombined by the lens at E. To 

 make an observation, the coincidence of the radiant point and its 

 image must be somewhat disturbed, the one being displaced to a 

 position a little beyond, and the other to a position a little in front 

 of, the diagram. 



The eye, protected from the flame by a suitable screen, is placed 

 at the image, and being focused upon A B, sees the field traversed by 

 bands. The reflector D is introduced as a matter of convenience to 

 make the line of vision horizontal. 



These bands may be photographed. The lens of the camera takes 

 the place of the eye, and should be as close to the flame as possible. 

 With suitable plates, sensitised by cyanin, the exposure required 

 may vary from ten minutes to an hour. To get the best results, the 

 hinder surface of A should be blackened, and the front surface of B 

 should be thrown out of action by the superposition of a wedge-shaped 



* The theory is given in a paper upon " Achromatic Interference Bands," 

 Phil. Mag. Aug. 1889. 



