It is to be used by candle-light in this manner: Attach to the fixed pins a small lock of the wool to be examined, containing 20 or 30 single fibres, and look through them and through the central hole at a candle not very remote from the card, the blackened side of the card being turned towards the eye. The hole will appear to be surrounded by a bright surface, reddish at the margin, by a dark circle or ring, and again by a brighter ring, bluish-green within and red without. If the colours are not seen distinctly, we may conclude that the wool is mixed and not perfectly fit for examination; but in this case we may generally form an estimate of its quality by means of the bright surface only, moving the card along the rod until the holes of the inner circle appear exactly at the extreme margin of that surface, where it is of a reddish hue, the place of the card as indicated by the scale showing the number which characterises the wool. It will also be most convenient to begin by producing this coincidence of the inner circle in other cases where the outer ring of colours is more distinctly seen, and then to adjust the card more accurately; so that the holes in the circumference of the outer circle may appear to coincide with the middle of the coloured ring at the common limit of the red and greenish portions. If the holes be seen in the red ring, the card must be brought nearer to the edge; if in the blue, it must be moved farther off, and the number of the scale must be observed as before. It will require very attentive observation, and perhaps some practice, to obtain always precisely the same number for the same substance, but by taking the mean of several trials we may be perfectly certain of coming within a unit of the true place of the slider, and perhaps even of avoiding any error at all. I have not yet had an opportunity of examining any great variety of substances, but I send you the result of such observations as I have made, which will be sufficient to enable you to judge of the accuracy of the instrument. Fibres of coarse wool from green baize, 52. Southdown, 35. Anglo-merino from a flock of Mr. Henty Tarring, 27. A lock taken from a Paular ewe by Mr. Sheppard, 25 (varying from 24 to 30). Another specimen from the same flock brought by another gentleman as a test of the instrument, 25. Cotton, mixed but about, 19. Vigonia from the Rev. P. Wood, very distinct 14½. Beaver from a hat, 11. Blood diluted with saliva and rubbed on glass, beautifully distinct 5. Milk diluted with water, very indistinct 3. I have sometimes thought of employing the instrument as a nosological test of the state of the blood, of pus, and of other animal fluids.