The probable error of a series of ten readings of fine print, under the actual conditions of observation with a (feeble) standard luminosity, is determined for two of the observers as follows: The measurements with violet light were made June 19, 1888, "sky hazy blue, thin but uniform cirrus haze." Those at wave-lengths 0.60 and 0.65 were obtained on June 20, 1888, "sky hazy blue with cumuli, haze not as dense as on the 19th but possibly less uniform." For a large part of the spectrum the probable error of a single reading does not exceed 4 per cent but the error may considerably exceed this for the violet rays, the eye requiring a much longer time to regain its sensitiveness for light of this color than for any other, so that for measures in this region an hour's stay in the darkened room is none too much to develop the full power of an eye which has recently been exposed to the full sunshine. Time required for Vision. In connection with the photometric measures the time required for the perception of very faint colored lights was Photometry by the reading of fine print gave for the same observer Violet, sensitiveness of eye =0·104,000 unity being the sensitiveness for yellow light; and the relative effect by this method is investigated. The method was an electrical one. There was automatic registration on a chronograph of the instant of exhibition, and determination of the instant of response as the observer pressed a key. The interval of course includes quite a train of distinct operations. According to Mendenhall (this Journal, III, vol. ii, p. 156), that portion of the action of brain nerve, and muscle which produces the mechanical effect, and which may be called automatic, takes place in certainly but little over one-tenth of a second. But the sensations which demand a conscious concentration of the attention, and especially those which require for their registration a decision of the judgment, occupy an interval several times as great. The perception of a light just at the verge of visibility probably involves an exercise of judgment,-an answer to the question, "Do I see the light or do I not ?"-although the question may not be consciously propounded, and accordingly this kind of perception may be included in that class of combined sensation and mental operation which involves a choice. Professor Mendenhall found for the time required to decide between red and white 0sec. ·443 and to decide between a circle and a triangle 0sec. 494. We have found for the average of over 1000 observations of the disappearance or reappearance of a very faint light (perhaps 20 times as bright as the faintest perceptible), ()sec. 507, but corresponding measures with a moderately bright spectrum, the light being about 10,000 times as intense as that called "very faint," gave Osec. 242, a number which is intermediate between the times found by Professor Mendenhall for the appearance of a white card (0sec. 292) and that of an electric spark (0sec. .203). We may therefore conclude that distinct vision for a very faint light demands about one half second of time, while the perception of light of ordinary brightness requires only about half that interval. It is possible that differences in the rapidity of the perception for lights of different colors might be detected on more exhaustive study, but none have been noted in these experiments other than those which were attributable to the variation of intensity. It will be seen that quantitative measures of the effect upon the eye of different rays whose luminosity varied in the proportion of 200 000 1, were actually obtained and that it would have been possible to considerably exceed these limits, especially when it is considered that the photometric measures were confined to lights of feeble intensity. Since it is possible to look directly at the sun for as short a time as one-half second, it is certain that the eye, by the combined adaptability of the iris and retina, can perceive lights whose intensities vary in the ratio of 1 to 1 000 000000 000000.* (10)1 *It may be interesting to check this result by an entirely different method. The light of the sun is, according to Pickering, equal to that of a star of —25′5 It will be understood that the writer does not profess any competence in physiological optics, and that the preceding observations and the conclusions reached from them are both to be understood from the purely physical point of view. This being premised, we will summarize the paper in the following conclusions. The time required for the distinct perception of an excessively faint light is about one-half second. A relatively very long time is, however, needed for the recovery of sensitiveness after exposure to a bright light, and the time demanded for this restoration of complete visual power appears to be greatest when the light to be perceived is of a violet color. The visual effect produced by any given, constant amount of energy varies enormously according to the color of the light in question. It varies considerably between eyes which may ordinarily be called normal ones, but an average gives the fol lowing proportionate result for seven points in the normal spectrum, whose wave-lengths correspond approximately with those of the ordinary color divisions, where unity is the amount of energy (about erg) required to make us see light in the crimson of the spectrum near A, and where the six preceding wave-lengths given correspond approximately to the six colors violet, blue, green, yellow, orange, red. Color. Violet. Blue. Wave-length, ".40 μ.47 Green. Yellow. Orange. Red. Crims. •47 -53 #.58 Luminosity, 1,600 62,000 100,000 28,000 (Visual effect.) Since we can recognize color still deeper than this crimson, it appears that the same amount of energy may produce at least 100,000 times the visual effect in one color of the spectrum that it does in another, and that the vis viva of the waves whose length is 0.75, arrested by the ordinary retina, represents work done in giving rise to the sensation of crimson light of 0.0000000000003 horse power, or about 0·001 of an erg, while the sensation of green can be produced by 0.000000,01 of an erg. øtellar magnitude, or 4400,000000 times that of Sirius (Mag. -14) which again is about 910 times that of a sixth magnitude star, ordinarily considered the faintest visible to the naked eye. Here the light of the sun is to that of the minimum visibile as 1 to 4,000000,000000 (4 × 1012), but the difference seems accounted for by the fact that the ratio by this latter method is found for an eye exposed in starlight by the former for an eye in absolute darkness. ART. XL.-Mineralogical Notes ;* by W. EARL HIDDEN. Xenotime, from New York City.-In 1872 I discovered, in the vicinity of 155th street and 11th avenue, a single crystal of a mineral which I then determined to be xenotime. It occurred in a coarse pegmatite vein traversing gneiss and associated with much muscovite, tourmaline, orthoclase, quartz and apatite. The determination was based upon the form and physical characters which agreed with those of xenotime. This crystal was inch in diameter. It showed the planes 1 (111) and I (110) and in habit resembled figure 438 of Dana's System of Mineralogy. The planes were too uneven to allow of exact measurement. In February of this year my attention was called, by Mr. William Niven, of New York, to a series of specimens he had lately collected in the same neighborhood. They included besides black tourmaline, dark green apatite, muscovite and orthoclase, two small brown crystals on separate specimens resembling zircon in form. An indication of prismatic cleavage and the inferior hardness (about 5) led me to believe them to be xenotime; this determination was confirmed by a chemical test of one of the crystals which yielded a large amount of P,O,.. These new crystals have long prisms and very smooth planes, thus differing from the first one found in 1872. The color is dark hair-brown. It is similar in form to a crystal from Schüttenhofent figured by R. Sharizer, excepting that it is much longer vertically. It is similar also to the xenotimes from Alexander County already described in this Journalt and to crystals from Hittero, Norway lately described by G. Flink.§ Careful measurements were made by Mr. H. S. Washington, with a Fuess horizontal goniometer, of the angles of one of these New York Island xenotimes. He states that, "the images were very good and the angles are quite reliable and useful for comparison. They agree well with vom Rath's values." following were the best angles (supplement) obtained. The Only three planes were observed, i. e., s (111), m (110) and r (311). * Continued from vol. xxxiii, p. 501. "Zeitschrift für Krystallographie, etc.," xiii, 1, 1887, figure 1. This Jour., III, xxxii, p. 206, Sept., 1886. Bihang till K. Svensk. Vet. Akad. Hand., xii, part 2, No. 2, p. 41 (1886). Jahrb. Min., 1879, p. 536. Later work at this locality, by Mr. Nivens, has resulted in the finding of crystals of zircon (G. = 4·73), monazite (G. = 5·51), chrysoberyl, iolite, pinite, and two others, which I have not as yet identified, all new to the locality. 2 Xenotime, from Alexander Co., North Carolina.-In some concentrations of vein material from the locality formerly known as Milholland's Mill,* I have also found several brilliant brown transparent crystals of xenotime. An ounce of this sand yielded more than a dozen perfect crystals. They were minute, none of them exceeding 2mm in length and from to mm in thickness. The hardness was less than 5; the quantity was too minute to allow of a specific gravity determination. With these hairbrown xenotimes and topaz-yellow monazite also occur very brilliant ruby-red crystals of rutile and small muscovite crystals. The measurements of the angles were attended with considerable difficulty but the following satisfactory results were finally obtained: s′ ^ s’'' (111 ~ ÏÏ1) = 964° approx. As to the new plane ƒ (2-i, 201), it was observed equally developed, as shown in figure 1, on all the crystals found. Its symbol is determined without measurement, by the fact that its terminal edge is truncated by the pyramid 1. It is to be hoped that the mining now going on in this region may bring to light macroscopic examples of this very interesting type of xenotime. At a new locality, about 3 miles nearly due east, I found, in August, a few small crystals of xenotime of a dark brown color that had polished planes. They were found associated with monazite, rutile, muscovite and quartz crystals. Xenotime-zircon, from a new locality.-On the Davis land, on the east side of the road that leads from Zirconia Station to Greenville, via. "Poinsetts Spring;" and distant about four miles from Green River Post Office, Henderson County, North Carolina, there is an outcropping of decomposing granite that has lately yielded a small quantity of very interesting minerals; only one of which I shall describe at this time. A cubic yard of the partly kaolinized material yielded on washing nearly an ounce of a mixture of zircon, monazite, xenotime, a member (as yet unidentified) of the samarskite group, and considerable magnetite in octahedral crystals. Figure 2 shows the symmetrical development of the largest xenotime found, and its association, in parallel position, with a crystal of zircon. The specimen is nearly one centimeter thick. The zircon crystal forming the center is bright dark-brown and the xeno * Now Warren's. On the monazite from this locality see this Journal, xxii, 21, 1881, and xxiv, 247, 1882. |