216 Mr. W. B. Cartmel on the Anomalous 



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 to 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 carelessly 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 possible for 

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

 metres away was observed by means of this telescope in such 

 a way that light from the distant object reached the telescope 

 directly and at the same time by reflexion 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 w\as 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 ob- 

 serving with the naked eye the reflexion of a pointed object 

 held near the instrument in such a way that reflexions of the 

 object reached the eye by way of the two paths of the instru- 

 ment, and the images thus seen brought into coincidence by 

 moving one of the mirrors parallel to itself by a screw motion. 

 When the imao-es are thus brouo-ht into coincidence the two 

 paths of the instrument are equal, and the coloured fringes 

 of white light can 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 cannot 

 be so widened or narrowed or rotated through any azimuth 

 if the mirrors are not nearly parallel. 



After having arranged the interferometer so that ihe fringes 

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

 perienced in seeing them with a telescope, but there was still 

 trouble in seeing spectral bands when the apparatus was set 

 up as shown in PI. IX. fig. 1. The difficulty was found to be in 

 the lenses L and L', which, though achromatic and well cor- 

 rected, were of verv short focus : but on chano-ino- these for 

 lenses of 25 cms. 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 Idnd of spectroscope would 

 give the best results when viewing very faint bands, a trial 



