REVERSED AND NON-REVERSED SPECTRA. 113 



the general procedure. In case of figure 74, the mirror N' must be high and 

 M' low. This is usually less convenient than the case when both mirrors 

 are high (C placed at G] or where both mirrors are low (C f placed at G'}. 

 In the former case, again, the rays have considerably diverged in a vertical 

 plane and the fringes are less marked. If C' is at G' the whole of each com- 

 ponent beam may be caught and passed through the respective shanks of 

 the U-tube. The fringes are strong, easily found, and large, so that the cen- 

 ter of ellipses is not far outside of the field of the telescope. It is obvious 

 that to facilitate adjustment the mirrors m and m', figure 71, must be nearly 

 parallel. They are made so by the aid of a broad beam of sunlight and then 

 clamped firmly in position at about 45 to the respective axis of the U-tube. 



Finally, if the connecting-tube p is nearly horizontal when in place, the 

 fringes are usually found at about the same position of the micrometer (at 

 M') after the liquid is introduced into the U-tube. Here it is advantageous 

 and usually permissible to make a part of the connection p of flexible rubber 

 tubing. But unless the free surface w, figure 71, is very nearly parallel to 

 the plate e, the center of ellipses is liable to be far outside of the field of the 

 telescope and the fringes correspondingly small. The difficulty of adjust- 

 ment for large fringes is now considerable, because of the two mobile liquid 

 surfaces in the U -tubes. For this reason I did not attempt to make measure- 

 ments, although the fringes themselves were surprisingly steady and strong 

 and would have been quite available, apart from laboratory tremors. Slight 

 changes in density, due to solution or temperature changes in one shank of 

 the U-tube, were well recorded, after stirring, with curious effects of surface- 

 tension and viscosity. 



The fringes being very clear, a number of other promiscuous experiments 

 were tried. Thus, a tube with plate-glass ends and filled with water was made 

 the core of a powerful magnetic helix. The tube, 26 cm. long, was placed 

 in one of the component beams and compensated by a column of glass in 

 the other. Good fringes were easily found ; but not the slightest displacement 

 could be detected by alternations of presence and absence of the magnetic 

 field. The water was now replaced by a solution of nickel sulphate. Fringes 

 were again easily found and strong in the green, but the effect of the magnetic 

 field was quite as inappreciable as before. Magnetic fields were thus totally 

 ineffective. 



In a set of experiments of a different kind the attempt was made to observe 

 the gradual deposition of silver on plate-glass. Bottger's solutions were 

 poured into a plane-parallel clean glass trough normal to one of the component 

 beams (cd, fig. 74) and compensated by a plate of glass in the other (bd). 

 Large fringes were produced by setting the micrometer and observed during 

 the formation of the two silver films on the opposite faces of the trough, until 

 they became quite opaque. It was astonishing to find that fringes were still 

 faintly visible long after a highly reflecting mirror had been deposited. But 

 no displacement larger than a fraction of a fringe could be detected, showing 

 the extraordinary thinness of the silver film even when practically opaque. 



