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 he 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.
4. Roughness.
The threshold for the appreciation of roughness is most conveniently determined by the Graham-Brown (ae)sthesiometer. This consists of a mass of brass with a polished surface, from which a tooth may be projected by means of a graduated screw. The instrument is drawn firmly over the part to be tested, and after each application the tooth is projected further until the patient can recognise the roughness. When the threshold is normal this is generally apparent to the observer's fingers holding the instrument at the same time as to the patient. The tooth "rakes" the skin, and this stimulus is conveyed both to the observer and the sense organs of the patient, provided his sensation is normal. Throughout this work we have used the original form of the instrument with one projection rather than that with many projecting cylinders.
We have used for the same purpose emery- or glass-paper of different degrees of roughness. We have adopted five grades, and employ as a control a piece of smooth cardboard of the same thickness. The normal fingers, when drawn over the rough surface, can recognise even the finest emery-paper we employ as rough, and can easily distinguish the relative roughness of any two grades. When this form of sensation is affected the finest grade that can be recognised as rough represents the amount of the defect. This is a useful test in cases of cerebral lesions, for by it the power of discriminating the relative roughness of two grades tested in succession may easily be determined.
