441 
1888.] Mr A. Johnstone on Carbonic Add Water. 
the weathered varieties, varieties which have been altered in 
consequence of exposure to water plus carbonic acid and air, the 
cracks of the hrachydiagonal cleavage, at least, are usually tolerably 
distinct. 
5. Is the Law of Talbot true for very rapidly Intermit- 
tent Light? By George N. Stewart, Esq., Senior 
Demonstrator of Physiology^ Owens College, Manchester. 
The law which is sometimes associated with the name of Talbot 
is generally stated thus : — Once complete fusion has been reached, 
no alteration in the intensity of the resultant impression produced 
by a series of flashes takes place, however short the time during 
which each flash acts may be, provided that the number of flashes 
in a given time and the length of each stimulation be always kept 
inversely proportional. Complete fusion of stimuli here is analo- 
gous to complete tetanus of muscle. And, as in the latter case, it 
has been discussed as to where the upper limit of frequency lies, 
or whether there be an upper limit, so in the former case are 
like questions in place. With the various answers which have 
been given in regard to muscle tetanus we are not here concerned ; 
except that it may be noticed that the later investigators, where 
they have at all admitted the probability of a limit, have had 
a tendency to shorten the time between each stimulus which 
they regarded as the minimum. 
The analogous question for retinal stimulation may be stated a 
little more fully. It is this : Granting that so long as the individual 
stimuli are effective the law of Talbot is true, is there any limit of 
time below which the individual stimuli cease to affect the retina at 
all, even when the frequency of repetition increases in proportion 
to the diminution of the time during which each acts ? In other 
words, is the retinal tetanus a complete tetanus, however short the 
duration of each stimulus % This is not the same thing as to ask 
whether there is a minimum time during which a single isolated 
stimulus must act in order to call forth a sensation. There is 
certainly such a minimum. It lies lower the stronger the light, 
and above this limit and below another the physiological intensity 
