Carrying Electrical Current. 105 



braced with slight strain against rotation around the cross 

 arm. In the same way J^"must be laterally braced with slight 

 strain against rotation around the arm under N or NP. 



The arm of the telescope R, in fig. 1, is in this case turned 

 about P, the telescope being simultaneously rotated until it 

 occupies the position D in fig. 2. It is for this reason that 

 two telescopes are used, as it would be much less convenient to 

 provide a special axle moving about the foot of G. 



To adjust the apparatus the arms ml and n' are first made 

 roughly equal with a scale. P and G are then placed in the 

 same line by a straight edge, the slit is widened so that the 

 beam m passes directly through central parts of P and the 

 reflection (spot) from the central part of the mirror M is seen 

 to strike the central part of the grating G. Proceeding thence, 

 the beam is reflected into the telescope T, approximately in 

 position (both M and G being rotated horizontally and verti- 

 cally for this purpose, finally by the adjusting screws). Next 

 the beam reflected from P is made to strike the central parts 

 of the mirror JV (which it should do at once on very slight 

 rotation of P), from which it is reflected to the central parts of 

 the grating G, so that the two beams m' and n' may be across 

 the same vertical line. Finally, G is adjusted until the two 

 direct images coincide accurately (both horizontally and verti- 

 cally) in the telescope, a fine hair having been drawn across 

 the slit, as above. 



Under these circumstances there will be four direct images 

 in the telescope at P, due to front and rear reflection at P and 

 G, neither of which is optical plate ; but there will be but three 

 spectra or three sodium lines visible at P, since the ray 

 reflecting the rear face of G is not diffracted. To obtain the 

 interferences, a single spectrum line from M must be placed 

 fully in coincidence with either of the two lines from JV ; but 

 to find them is nevertheless a matter of considerable difficulty. 

 Since reflection takes place not from one and the same face of 

 glass, but from the two independent faces P and G, the ellip- 

 ses in the field of view are liable to be very eccentric. They 

 thus appear as the merest hair lines, easily overlooked even 

 when M has (by trial) been moved into the correct position. 

 In fact, if the sodium lines be called 



Na m , Na„, JVa 



n» ) 



I have only been able to get interferences from two of these, 

 say JVa m , JYa n , but not from JVa m , Ma a ,, after wasting much 

 patience in the attempt. There may be some other reason 

 for this which has escaped me. JVa m passes through three 

 thicknesses of glass, whereas JVa n and JVa a , pass through one 

 and three thicknesses, respectively, and I have not been able to 



