The Dispersion and Absorption of Fnchsin -3 



introduced in one of the paths and an absorbing screen in the 

 other. Good fringes may now be seen because the intensity of 

 the Hght in tlie two paths is reduced. The retardation may be 

 measured by the shift of the fringes on the removal 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 absorb- 

 ing screen should have only half the absorption of the fuchsin 

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

 ness whether the fuchsin film is in or out 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 dis- 

 placed with respect to the other. 



Sunlight from a slit S, fig. i, 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 millimeter wide, a portion narrow enough so that the 

 thickness could be determined definitely. With a wider film 

 only an average thickness could have been obtained. 



The Hght, 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 eye-piece of the spectroscope, 

 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 bands might, under 

 certain circumstances, be seen with the eye in one position, but 

 with the eye nearer or farther from the interferometer they were 

 invisible. Or again, by moving the eye nearer or farther away 

 from the interferometer, a position might be found in which two 

 sets of bands could be seen, crossing one another at right angles, 

 though this only occurred when the instrument was very care- 

 lessly adjusted. The following method of adjustment enabled 

 very satisfactory fringes to be produced. A telescope having a 

 fairly well corrected objective was focussed as carefully as pos- 



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