It is thus seen that totally colour-mistaken people are not so exact in adjusting as the singly colour-mistaken,-possibly from a little hastiness if they are tired by the investigation, but partly from amblyopia and atrophy* of the visual nerve, which probably always accompanies so high a degree of colour-disease. This is also seen in the circumstance that the position of the lower Nicol varies within certain limits. These cases can thus be characterized by a group of equations which indicates the results for both limits, and a mean value of the lower Nicol; thus, for instance, for U+30=0±90±; +41— Mr. O, U+45=0+20-; -73+ Remembering that Daltonists may be at the same time colourblind-indeed blind to red and to violet, and both to different degrees-it is easy to conceive that no Daltonist sees colour exactly like another, and what different perceptions of colour the same excitation must produce in different individuals. Among fifty-nine colour-diseased I have not found any two who exactly exhibit the same sense of colour. It seems almost a matter of course, and it follows, indeed, from other experiments and reasoning, as I showed some years agot, that Young's theory of colour, whether modified or not, is irreconcileable with this; for it only allows three kinds of colourblindness, or, taking in the combinations, six. * This is the reason why I am not certain whether Mr. K is affected in the same way as Messrs. L and O; for each time I have had to abstain from further investigation. † Gräfe's Archiv, 1860, p. 89; and Virchow's Archiv, vol. xx. p. 282. XII. On the Bands formed by the Superposition of Paragenic Spectra produced by the Grooved Surfaces of Glass and Steel.-Part II. By Sir DAVID BREWSTER, K.H., F.R.S.L. & E.* IN [With Two Plates.] N the preceding paper, I have described the bands produced by gratings or grooved surfaces with 500 divisions in an inch when the two grooved surfaces are in contact, and the grooves in the one slightly inclined to those in the other. The following results were obtained with two gratings, one of which had 2000 and the other 1000 divisions in an inch. 1. When the surfaces are in perfect contact and the grooves parallel, the bands seen on the united surfaces, either with a lens or by ordinary vision, are very irregular and are parallel to the grooves. They are seen only on the 2nd, 4th, 6th, &c. spectra on each side of the luminous bar or disk. By turning the nearest grating slightly to the right from the azimuth 0°, the bands fall back to the left, increasing in number, and descending with their concave sides downwards into distinct serrated black and white bands nearly perpendicular to the grooves. When the nearest grating is turned to the left, the bands descend towards the right, with their concave sides upwards, till they become nearly perpendicular to the grooves. In all these positions the bands are twice as numerous on the fourth spectrum as on the second, and thrice as numerous on the sixth as on the second; and when the grooved surfaces are perfectly parallel, the bands are immoveable on the grooved surfaces at all angles of incidence. 2. When the grooved surfaces are separated by the thickness of one or both of the plates of glass, the bands are very indistinctly seen, and they seem to diminish in size with the distance of the grooved surfaces; but this is not certain, owing to the difficulty of fixing the plates with the grooves at the same inclination to each other. Similar bands were seen on the united surfaces of gratings of 2000 and 2000, 1000 and 1000, 500 and 500, 1000 and 500, and 2000 and 500 divisions in an inch, but always less distinctly when the grooved surfaces are separated by the thickness of one or both of the plates. The beauty and distinctness of these bands depend upon the skill with which the gratings are ruled. In several of the gratings which I possess, the phenomena I have described can hardly be recognized. When the combined gratings have the same number of divi*From the Transactions of the Royal Society of Edinburgh, vol. xxiv. |