Dispersion and Selective Absorption of Fuchsin. 215 



After a number of trials to determine the best adjustment 

 it was found possible, by using this type of interferometer 

 and making the films sufficiently thin, to obtain distinct 

 fringes throughout the spectrum. An unsymmetrical ar- 

 rangement was first tried, in which the partly silvered plates 

 were very lightly silvered or not silvered at all, and the 

 fringes observed in a direction at right anoies to that at 

 which the light entered the instrument. In this way fringes 

 were obtained from beams of unequal intensity, though the 

 method was finally abandoned for the following, which is 

 more satisfactory. 



The partly silvered mirrors of the interferometer are 

 silvered so as to reflect and transmit equally, in order that 

 the light in the two paths may be of equal intensity, and then 

 the fuchsin-film is introduced in one of the paths and an ab- 

 sorbing screen in the other. Good fringes may now be seen 

 because the intensity of the light in the two paths is re- 

 duced. The retardation may be measured by the shift of* 

 the fringes on the remoA^al of the fuchsin-film. But the 

 presence of the absorbing screen causes the fringes to be 

 indistinct when the fuchsin-film is removed, so the absorbing 

 screen should have only half the absorption of the fuchsin- 

 film, in order that the fringes may be seen with equal dis- 

 tinctness whether the fuchsin-film is in or out of the inter- 

 ferometer. In practice the fuchsin-film was placed in the 

 upper or lower half of one path of the interferometer, and the 

 absorbing screen in both upper and lower halves of the other 

 path, so that with the interferometer adjusted for vertical 

 fringes two sets were seen one above the other, but one set 

 displaced wdth respect to the other. 



Sunlight from a slit S (PI. IX. fig. 1) was brought to a focus 

 by means of the lens L, so that an image of the slit fell upon the 

 glass plate upon which the fuchsin had been deposited, and 

 thus light of very great intensity was concentrated upon a 

 strip of the film only a millimetre wide, a portion narrow- 

 enough for the thickness to be determined definitely. With 

 a wider film only an average thickness could have been 

 obtained. 



The light after leaving the interferometer was brought to 

 a focus upon the slit of a small spectroscope by means of the 

 lens L^ By observing through the eyepiece of the spectro- 

 scope spectral bands could be seen. 



At this point it may be as well to mention that it takes 

 very careful adjustment of this type of interferometer to be 

 able to observe the bands, either with a spectroscope or a 

 telescope. Even with the naked eye it was found that the 



