ON MINERS’ NYSTAGMUS. 243 
perifoveal sensation, in the absence of foveal sensation, or a perifoveal 
sensation of greater intensity than a foveal sensation, would excite 
a fixational or movement reflex. This would bring the exciting point 
in the marginal field on to the fovea, and would then either cease 
altogether to excite sensation or would be so diminished in intensity 
as to lay the eye open again to marginal stimulation. With a central 
stigma, a neurasthenic diathesis, there would be present all the material 
for the development of a habit spasm. 
A large number of observations were made on students, assistants, 
and ourselves with a piece of apparatus described in the appendix. 
This apparatus was arranged to present a spot of light the intensity of 
which was controllable. The open eye was directed on this spot while 
the room was in full daylight, and then the room was suddenly plunged 
into darkness. At the end of five seconds the subject was examined as 
to his ability with direct vision to perceive this faintly illuminated spot, 
its intensity rapidly altered until the subject was only just able to per- 
ceive it. The time for this, the minimum visibile, usually took about 
five seconds, and the degree of illumination was remarkably constant in 
all these cases, so much so that we were able to fix on this degree of 
light intensity as our zero. The direct vision was contrasted with 
indirect vision, the subject being directed to look slightly away from the 
spot of light, which at once appeared to become more vivid. The inten- 
sity was then diminished until here again the spot was only just dis- 
cerned, and so we obtained a minimum vision visibile for indirect or 
perifoveal vision. 
At intervals of two and a half minutes the minimum visibile was 
estimated for both fovea and perifovea, and we were able to represent 
in graphic form the increasing sensibility of the retina to faint illumina- 
tion. In general the dark adaptability of fovea and perifovea increased 
rapidly up to the end of half an hour, less rapidly up to the end of two 
hours ; arriving then at its maximum sensibility it remained stationary. 
The perifovea throughout this development of dark adaptation not only 
retained its primary advantage, but slightly increased that advantage up 
to the limit of change. In our experience in the coal-mines we never, 
however, felt that the maximum amount of sensibility was ever in 
demand, and while the light was indeed feeble enough to be exces- 
sively irritating to our unaccustomed eyes, yet nowhere did we find 
working conditions approaching our experimental conditions. Was the 
miner’s eye differently equipped from our own, did it possess a greater 
adaptability through use and habit ? 
This led to an examination of the dark-adaptability of miners who 
had been afflicted with nystagmus, and here we found without exception 
a totally unexpected condition, yet one which now rendered plausible 
our fixational hypothesis. Instead of finding a greatly increased adapt- 
ability as a result of long use and cultivation of the eye in the dark, 
we found that in this respect it was greatly inferior to the ‘ normal ’ eye. 
In the first place the ‘zero’ was not perceived until after about five 
minutes of exposure to the dark, and then once perceived it remained 
without an appreciable development through the two hours’ experiment. 
This peculiarity on the subjective side amounted to the same thing in 
A R 2 
