762 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
line. If this line is horizontal the bands will be horizontal, and for this direction 
the mirrors seem accurately plane. But for the vertical line of intersection the length 
of the mirrors is effective, and this seems to be slightly curved, so as to throw the 
focal plane of the vertical bands further out. 
This does not account for the hyperbolse. But with sodium light, systems of hyper¬ 
bolae, and also ellipses, can be seen in Brewster or Jamin plates, with different 
angles of setting (see Lummer, ‘Wiedemann’s Annalen,’ vol. 24, p. 417), and T expect 
the theory is somewhat similar. 
Fig. 10. 
One fi'eqnent appearance of the bands.* (But light and dark should be intercbanged in tbe figure.) 
36. The following observations were made recently as to the effect of various 
movements on each set of fringes :— 
* Cassini ovals are just as easy to get as these. The bands are always curves of the fourth degree 
and arc, of course, sections of surfaces of constant retardation. The virtual sources may be taken as 
in fig. 7, viz., the nearly coincident images, C and C". 
