170 M. G. Quincke on the Optical Properties 



matic lens. The rays emerged nearly parallel from the lens, and 

 then encountered a system composed of two plane-parallel plates 

 of glass of exactly equal thickness (8*66 millims.), and placed 

 200 millions, apart. The second plate of glass was a very little 

 inclined towards the first, so that the rays reflected from the 

 posterior surface of the first and from the anterior surface of the 

 second plate might interfere with the rays reflected at the ante- 

 rior surface of the first and the posterior surface of the second. 

 A screen cut off the rays reflected from both anterior or from 

 both posterior surfaces of the plane-parallel glass plates. The 

 rays admitted by the screen fell upon a flint-glass prism with a 

 refraction-angle of 60° and a vertical refracting edge, in which a 

 spectrum having thick black bands of interference could be seen 

 with the naked eye. By regulating the breadth of the slit and 

 the distance of the same from the lens, it was possible to make 

 Fraunhofer's lines clearly visible. The interference-bands were 

 caused to fall parallel to Fraunhofer's lines by a proper adjust- 

 ment of the plane-parallel plates of glass. 



In the following experiments, both interfering rays of light 

 were inclined at an angle of 45° or 30° to the anterior surface of 

 the plane-parallel glasses, and had a great intensity in this 

 arrangement of the apparatus. A very slight difference in the 

 path of the rays, even when it fell short of fo of an undula- 

 tion, could be recognized with certainty by an alteration in the 

 position of the thick interference-bands — especially when this 

 alteration was restricted either to the upper or to the lower 

 part of the spectrum, the other portion remaining unaltered. 

 Besides, the displacement could be observed against the Fraun- 

 hofer's lines. 



The section of each of the interfering rays (or, more cor- 

 rectly, bundles of rays) was very nearly a rectangle, the longer 

 side being vertical. In the path of both bundles of rays, which 

 were about 4 to 6 millims. apart, there was now placed a plane- 

 parallel glass plate, on which there was a rectangular transparent 

 metallic film. One side of this film was placed horizontal, the 

 other vertical, so however that the latter lay between the two bun- 

 dies of interfering rays. Assuming that the upper part of the 

 bundle of rays I. passed through metal, and that the upper 

 part of the bundle of rays II. passed through air, then the 

 upper part of the spectrum would be formed of rays which would 

 have a difference of path proportional to the thickness of the 

 metallic plate, if light travels through metal with a velocity dif- 

 ferent from that at which it travels through air. 



Since the metallic plate, and hence the upper part of the spec- 

 trum, was sharply bounded by a horizontal line, it was possible to 

 observe the situation of the interference-bands in the lower part 



