1893.] on Interference Bands and their Applications. 77 



difficulties are greatly reduced. If we superpose a layer of water 

 upon a surface of mercury, the flatness and parallelism of the surfaces 

 take care of themselves. The objection that the two surfaces would 

 reflect very unequally may be obviated by the addition of so much 

 dissolved colouring matter, e.g. soluble aniline blue, to the water as 

 shall equalise the intensities of the two reflected lights. If the ad- 

 justments are properly made, the whole field, with the exception of a 

 margin near the sides of the containing vessel, may be brought to 

 one degree of brightness, being in fact all included within a fraction 

 of a band. The width of the margin, within which rings appear, is 

 about one inch, in agreement with calculation founded upon the 

 known values of the capillary constants. During the establishment 

 of equilibrium after a disturbance, bands are seen due to variable 

 thickness, and when the layer is thin, persist for a considerable time. 



When the thickness of the layer is increased beyond a certain 

 point, the difficulty above discussed, depending upon obliquity, be- 

 comes excessive, and it is advisable to change the manner of obser- 

 vation to that adopted by Michelson. In this case the eye is focused, 

 not, as before, upon the operative surfaces, but upon the flame, or 

 rather upon its image at E (Fig. 2). For this purpose it is only 

 necessary to introduce an eye-piece of low power, which with the lens 

 C (in its second operation) may be regarded as a telescope. The 

 bands now seen depend entirely upon obliquity according to the 

 formula above written, and therefore take the form of circular arcs. 

 Since the thickness of the layer is absolutely constant, there is 

 nothing to interfere with the perfection of the bands except want of 

 homogeneity in the light. 



But, as Fizeau found many years ago, the latter difficulty soon 

 becomes serious. At a very moderate thickness it becomes necessary 

 to reduce the supply of soda, and even with a very feeble flame a 

 limit is soon reached. When the thickness was pushed as far as 

 possible, the retardation, calculated from the volume of liquid and 

 the diameter of the vessel, was found to be 50,000 wave-lengths, 

 almost exactly the limit fixed by Fizeau. 



To carry the experiment farther requires still more homogeneous 

 sources of light. It is well known that Michelson has recently 

 observed interference with retardations previously unheard of, and 

 with the aid of an instrument of ingenious construction has obtained 

 most interesting information with respect to the structure of various 

 spectral lines. 



A curious observation respecting the action of hydrofluoric acid 

 upon polished glass surfaces was mentioned in conclusion. After the 

 operation of the acid the surfaces appear to be covered with fine 

 scratches, in a manner which at first suggested the idea that the glass 

 had been left in a specially tender condition, and had become scratched 

 during the subsequent wiping. But it soon appeared that the effect 

 was a development of scratches previously existent in a latent state. 

 Thus parallel lines ruled with a knife edge, at first invisible even in 



