Dispersion and Selective Absorption of Ftichsin, 221 



a systematic error of some kind. A slioht clianoe in the 

 thickness woukl make the two curves ahnost identicaL 



The thickness of the fihns was determined as follows : — A 

 portion of the film was washed away with alcohol leaving a 

 clean sharp edge. In general, when a film is thus washed 

 away, there is a concentration of fuchsin at the edge, due to 

 a flowing of the solvent. This is of course undesirable, and 

 was eliminated by using a piece of blotting-paper with which 

 to clean away the fuchsin. This piece of blotting-paper was 

 itself cut to a clean sharp edge and slightly damped with 

 qJcohoL The glass near the film was wiped with it, a new- 

 edge of the blotting-paper was prepared, dampened, and the 

 operation was repeated until a satisfactory edge of fuchsin 

 was obtained. 



]N' ow a plate of glass was placed upon the glass upon which 

 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 thin 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 spectro- 

 scope, by means of which interference-bands could be seen 

 in the spectrum. 



The arrangement of the apparatus is shown by PL IX. 

 fig. 4. The glass plates inclosing the air-film are shown at G. 

 P 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 was 

 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 contrivance the plates were adjusted so that the 

 bands seen at the eyepiece 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 towards 

 the violet, those corresponding to the air-film between the 

 o^lass and the fuchsin showed an irreoifiaritv. 



* Wied. Ann., Fogg. Ann. Erglxl. viii. p. 65 (1878). 



