DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
761 
round caused them to shift. Introduction of ordinary sheet glass into the beam 
distorted them till they were like the water-mark on cloth. 
View of the optical frame, supporting the mirrors, telescope, and collimator: detached from its position 
round the steel disks, where it is shown in fig. 11. 
Later experience makes it absurdly easy to get the bands and to arrange that they 
shall be vertical, well defined, of any convenient width, and with the centre white 
band symmetrical among the coloured ones. Tilting the mirrors, pressing on the 
frame, or touching the semi-transparent plate, makes the coloured fringes move with a 
concertina-like motion towards or from the middle band, but the middle band is not 
easily shifted by anything. Altering the angle between the mirrors widens or narrows 
the bands, and when they get very wide a double system of hyperbolae usually makes 
its appearance. 
I conjecture that the horizontal bands first seen were the right or left branches of 
fig. 10, and that the widening or narrowing of the vertical bands may be expressed as 
an up or down motion of the figure. 
Sometimes by pushing in the eye-piece of the telescope another set of bands could 
be seen horizontal, in exchange for the vertical bands which had, as it were, gone out 
of focus. These horizontal bands were more tremulous than the others and tilted 
readily. They occurred in the proper focal plane for infinity (assuming the mirrors to 
be accurately plane), and are probably what I saw first. The vertical bands do not 
become visible till the eye-piece is pulled several inches out. The probable meaning 
of this double set is that the mirrors have a very slight cylindrical curvature, the 
generating line of the cylinder being vertical. A pair of opposite mirrors, though 
very nearly parallel, will intersect in a line, and the bands will be parallel to this 
MDcccxcm.— A, 5 E 
