Sentence:  If it be assumed ... (2) that the imperceptible value which it [resistance] finally attains is equal to the <WSD>"liminal</WSD> intensity" of the stimulus; and (3) that the intensity of the sensation is proportionate to the magnitude of the area of the circle of irradiation,—the same relation between strength of stimulus, <WSD>liminal</WSD> intensity, and intensity of sensation is found as is expressed by 'Fechner's formula' (Maassformel). 
Meaning of the word marked with <WSD> tags:  Of or pertaining to a beginning or first stage of a process. (from late 19th c.)