Abstract: 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 analogous 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.