J. H. Lane on an Automatic Comparison of Time. 47 
tinetly indicated in fig. 1, 6, and that the action of this differs 
in no respect from that of P except in its longer period of revo- 
lution, and that this period of revolution is the same with that 
of the signal wheel, it will be obvious that if several supplement- 
ary openings be made in the signal wheel, as for instance two, 
tand wu, in two teeth diametrically opposite to each other, and 
two others, v and w, in teeth adjacent to one of these on each 
side of it, the flashes of light through these openings will be 
seen by the eye at E to occur at points 7’, wu’, v', w’, fig. 3, distri- 
buted around the circumference of a circle concentric with 
h 
the fast prism. Instead of a pair of complete telescopes, the 
equivalent of a pair of eye-pieces with a sliding object-glass to 
alternate between them at pleasure, would answer the same pur- 
pose. In this arrangement no rectification of the prisms by the 
observer would be necessary, it being always possible to observe 
the total deviation. This would be a great advantage on a line 
of very numerous stations, in which case it would, on the first 
described plan, be a somewhat critical matter to bring all the in- 
struments on the line into the required correspondence for simul- 
neous observation. 
