12 W. B. Cartmel 



the fuchsin had been deposited. There were thus two layers of 

 air of different thickness, one between the fuchsin and the glass, 

 and the other between the two pieces of glass. The difference 

 in the thickness of the two films of air gives the thickness of the 

 fuchsin. The thickness of the air films may be obtained very 

 accurately by the interference of their films, by a method first 

 used by Wernicke^ and modified by Wiener. ^ White light was 

 allowed to fall on the two air films, and the reflected light was 

 examined with a spectroscope, by means of which interference 

 bands could be seen in the spectrian. 



The arrangement of the apparatus is shown by fig. 3. The 

 glass plates enclosing the air film are shown at G. F is a totally 

 reflecting prism, one edge of which forms an edge of the slit of 

 the collimator. A Rowland grating, R, sent a spectrum into the 

 observing telescope, and the distance from band to band was 

 measured by means of the micrometer eye-piece. 



The two glass plates, G, were so arranged before the slit that 

 the film of air between the fuchsin and the glass were before the 

 lower part of the slit, and the film of air between the two bare 

 plates before the upper part. By a suitable mechanical contri- 

 vance, the plates w^ere adjusted so that the bands seen at the eye- 

 piece were vertical. 



On examining the two sets of bands, it was noticed that while 

 those corresponding to the film between the two glass surfaces 

 were regular and became uniformly narrower toward the violet, 

 those corresponding to the air film between the glass and the 

 fuchsin showed an irregularity. 



For instance, when there was a film of fuchsin whose thickness 

 was about a fourth of a wave-length of red light, the bands of 

 one set were displaced with respect to the other set by half a 

 band in the red, and this displacement regularly increased, till 

 a point in the blue-green was reached where it was a whole band. 

 At this point there was a sudden change of half a band, and then 

 a regular change took place, going from short to shorter wave- 



^Pogg. Ann. Ergbd. 8, p. G5. 1878. 

 ^IVied. Ann., vol. 31, p. 629. 1887. 



112 



