in diameter. These were placed in a glass containing broken ice; on removal, each rod was carefully wiped and, after its flat base had been applied to the skin, was returned again to the ice. We found it best for H. to mark out the cold-spots over night, independently of any previous diagram or photograph. Next morning, they were tested by R., who confirmed or rejected these observations. After the spots had been charted, the result was compared with previous photographs, and if any spots of the first order did not appear, a further examination was made. At the end of the series of sittings, a photograph was taken of the final results (figs. 17 and 18). Occasionally, the back of the hand was protected by a glove, and the marks preserved for several weeks or even months.
For the discovery of heat-spots, we used a simple method which, as far as we can discover, has not been described before. We chose a "soldering iron" consisting of a large copper block fixed to an iron rod let into a wooden handle. This block, about 3 in. (7·5 cm.) in length and 1 in. (2·5 cm.) across every face, we cut down to a pyramidal point. The apex of the pyramid was flat and 1 mm. square. Into the copper block we bored a circular shaft, passing obliquely downwards in the direction of the point. This was of sufficient size to contain the bulb of a thermometer, just under 1 cm. in diameter.
Two of these irons were placed in a jug containing hot water. When sufficiently heated, one of them was removed and dried; the thermometer was placed in the cavity and the instrument laid on a cloth, until the required temperature was recorded. It was then held firmly in the hand like a large pen, and lightly applied, vertically, to the surface of the skin. So large a block of copper retains its heat for a considerable time, and the thermometer gives a sufficient indication of its temperature. This should lie between 50° C. and 40° C, preferably at about 45° C. Higher temperatures cause distinct pain, which complicates the observations; a temperature below 40° C. fails to stimulate most of the heat-spots.
A low external temperature greatly increased the difficulty in discovering both heat- and cold-spots; and in the winter, when the affected hand seemed numb and cold, previous immersion in warm water greatly facilitated their determination.
To test the power of discriminating two points we used a pair of carpenter's compasses, the points of which had been ground down until they gave no sensation of sharpness.[1] For recording our observations,
- ↑ Most of the instruments, called æsthesiometers, used for this purpose are provided with points so sharp that they arc wholly useless. The sharp ivory points of Spearman's instrument should be replaced by blunt ones.
