[FM] Chap. x. p. 202. [FN] The observations are not yet published, and I have to thank Lord Rayleigh for his courtesy in allowing me to make use of this fact. [FO] Professor Langley finds that the maximum effect with a radiating source at 170° C. is at about 5.0 thousandths of a millimetre wave-length. " 100° C. " " 7.5 " " " " 0° C. " " 11.0 " " " We are sensitive to radiations from a body at 100° C. But when the temperature falls below the normal temperature of the body we are not sensitive to heat-vibrations, but to loss of heat from the surface exposed. The limit of sensibility to heat-vibrations, therefore, probably lies between 7.5 and 11 thousandths of a millimetre. I have taken about 9.25 as the limit. CHAPTER VIII. MENTAL PROCESSES IN MAN. I have already drawn attention to the fact that the primary end and object of the reception of the influences (_stimuli_) of the external world, or environment, is to enable the organism to answer or respond to these special modes of influence, or stimuli. In other words, their purpose is to set agoing certain activities. Now, in the unicellular organism, where both the reception and the response are effected by one and the same cell, the activities are for the most part simple, though even among these protozoa there are some which show no little complexity of response. Where, however, the organism is composed of a number of cells, in which a differentiation of structure and a specialization of function have been effected, certain cells are set apart as _recipients_, while other cells are set apart to respond (_respondents_). There is thus the necessity of a channel of communication between the two. Hence yet other cells (_transmitters_), arranged end to end, form a line of connection and communication between the group of receiving cells and the group of responding cells, and constitute what we term a _nerve_. That which is transmitted may still be called a stimulus, each cell being stimulated in turn by its neighbour. Thus a stimulus must be first received and then transmitted.