The Dispersion and Absorption of Fuchsm 5 



sible for parallel rays, and then an object at least two or three 

 hundred meters away was observed by means of this telescope 

 in sucii a way that light from the distant object reached the tele- 

 scope directly and at the same time by reflection from two of the 

 mirrors of the interferometer. If the mirrors were not quite 

 parallel, two images of the object could be seen, so that all that 

 was necessary to make the mirrors parallel was to adjust them 

 till the two images seen in the telescope coincided. In this way, 

 by comparing one mirror with another, all the mirrors of the 

 instrument could be brought into almost perfect parallelism. 

 The final adjustment was made by observing with the naked eye 

 the reflection of a pointed object held near the instrument in such 

 a way that reflections of the object reached the eye by way of the 

 ; ■ o paths of the instrument, and the images thus seen brought 

 into coincidence by moving one of the mirrors parallel to itself 

 by a screw motion. When the images are thus brought into co- 

 incidence, the two paths of the instrument are equal and the 

 colored fringes of white light may be seen. A simple adjustment 

 of one of the mirrors by trial will widen or narrow down the 

 bands at will, or rotate them through any azimuth, though they 

 can not be so widened or narrowed or rotated through any azi- 

 muth if the mirrors are not nearly parallel. 



After having arranged the interferometer so that the fringes 

 were all that could be desired, no difficulty whatever was experi- 

 enced in seeing them with a telescope, but there was still trouble 

 in seeing spectral bands when the apparatus was set up, as shown 

 in the figure. The difficulty was found to be in the lenses, L 

 and L' , which, though achromatic and well corrected, were of 

 very short focus, but on changing these for lenses of 25 cm. focal 

 length, the spectral bands were so bright and clear that they could 

 evidently be very much dimmed by the introduction of the fuchsin 

 and still be visible. 



In order to determine what kind of spectroscope would give 

 the best results when viewing very faint bands, a trial was made 

 using various spectroscopes. The conclusion reached was that a 

 spectroscope of very low dispersive power, having a low power 

 eye-piece, was the most satisfactory, and therefore that kind was 

 used in this work. 



10; 



