REVERSED AND NON-REVERSED SPECTRA. 141 



vertical achromatic fringes. The fringes are found with a few turns of the 

 micrometer after the liquid w has been poured in. 



The fringes as seen through the liquid columns are still strong and clear, 

 with scarcely any deterioration; but unfortunately in this locality they are 

 in incessant and vigorous vibration. It is indeed astonishing that a phenome- 

 non so sensitive can survive such relatively rough treatment. 



With this apparatus the endeavor was again made to determine the sus- 

 ceptibility of water. Good fringes were first produced, but for all levels of 

 the water-surfaces the effect of the presence or absence of a magnetic field 

 was quite nil, so far as any appreciable displacement of fringes was concerned. 

 They remained in place while the current in the helix was alternately closed 

 and opened. The reason for this completely negative result I am unable to 

 explain. 



In a repetition of such experiments tubes much wider than 4 cm. should 

 be used. At this diameter the liquid surfaces still show appreciable curvature, 

 which is an annoyance. 



The yoke, figures 96 and 97, is finally a considerable convenience in the meas- 

 urement of vertical angles near the zenith. The rotation of the mirror m' around 

 its two axes is here available. Horizontal achromatic fringes in all these cases 

 may often be produced and used to advantage. In such a case the two 

 adjustment screws of the mirror m' change their function, and the fringes 

 travel up and down the wide slit image with changes of AA/". As this image is 

 more extended vertically than horizontally, the fringes are much longer in 

 sight. Fringes are largest when the planes of symmetry of pencils of rays 

 accurately retrace their paths. But as large fringes are apt to be irregular, 

 a small angle of reflection at the mirror m' is preferable. 



73. The displacement interferometry of long distances. In the preceding 

 paragraphs I suggested two methods for the measurement of small angles. 

 The first (fig. 88) used an auxiliary mirror, and, apart from corrections, the 

 angle A a over which the auxiliary mirror m turns is 



(i) Aa = AA/"cos t'/2.R 



where AAT is the displacement of one of the plane mirrors parallel to itself 

 necessary to restore the achromatic fringes to their former position in the 

 field of the telescope, i the angle of incidence (conveniently 45), 2R the 

 normal distance apart (06 or cd) of the (parallel) interfering pencils in the 

 fore-and-aft direction of the incident beam. In the second method (fig. 93) 

 the auxiliary mirror is dispensed with and the rotation of a rigid system of 

 paired mirrors is used. The sensitiveness is half the preceding. 



Suppose that the paired mirrors near the telescope (figs. 88, 93) confront 

 but a part of the area of the objective and that the telescope can therefore 

 look over the mirrors directly into the region * beyond, as shown at K. The 



* A series of small mirrors or reflecting prisms may be employed to the same purpose; 

 or the mirrors may both be half-silvered and transparent. 



