TRANSACTIONS OF SECTION I. 903 
and 5 centimetres broad, painted white in front with a grey centre gradually 
diminishing in shade to white towards the extremities, This is revolved at its 
centre by an electric motor provided with a means of counting its revolutions, and 
worked by means of a five-cell electric storage battery. The lath is placed 1 
metre 14 centimetres from the nozzle of a projection lantern, in which there is a 
lantern slide representing a statue or other definite figure. On the lath at rest a 
small portion of the lantern image is projected and focussed. When the lath is 
revolved rapidly at about 318 revolutions per half-minute, the whole picture appears 
standing out boldly in space and in relief, and at this rate of revolutions is steady. 
The persistent image, however, is visible, though not steady, at much lower rates 
of revolution, the lowest rate being a matter of individual capacity. By ascer- 
taining the rate of revolutions of which each person can just see persistence his 
particular capacity is calculated. 
Tables containing the results of a hundred tests with 67 persons were shown. 
One of these gave a group of 25 tests with persons of both sexes and various ages 
and classes. The low rate of 27 revolutions per halfminute was taken as the 
lowest limit for the tests. 
Another table of special interest gave tests of 26 schoolboys before and after 
the bodily fatigue produced by running. The persistence of every subject was 
altered by running, except one, who registered the same in each case. The record 
of seven was lowered, but that of nineteen was heightened, showing that bodily 
oe tends to prolong persistence of vision, which seems to be the tendency of 
illness. 
A third table showed tests under the light of different colours ; the rays from the 
Jantern passing through red, green, and violet glasses. It also showed tests of 
three persons before and after retinal rest. 
The question of persistence of vision in relation to its duration in different 
individuals and in the same individual under different circumstances, appears to 
have an important bearing on modern rapid visual signalling. In reading the 
signals the signaller has to discriminate between real and incidental images, and 
his sharp reading of dots and dashes will depend upon the persisting capacity of 
his retina. A good signaller is likely to be one whose persistence of vision is 
abnormally low, a bad signaller one whose persistence of vision is abnormally 
high. The aerial graphoscope affords a means of testing signallers as to their 
capacities of persistence of vision, and of selecting the fittest. For this purpose 
it has lately been installed in the school of signalling at Aldershct. 
