Meteor Trains IN the Popular Science Monthly, August, 1911, Prof. C. C. Trowbridge, of Columbia University, presents a summary of the results thus far obtained in his study of meteor trains, which he has now carried on for a number of years, and which he has made the subject of several earlier contributions to the scientific journals. His activity in this field of research has been recognized by the National Academy of Sciences, which has made him a grant from the J. Lawrence Smith Fund, to enable him to extend his work in this direction. Prof. Trowbridge applies the term persistent train to a meteor trail of long duration-from several minutes to an hour 0' more-as distinguished from a trail lasting, at most, a second or two, such as is ordinarily seen in the wake of a meteor. A large number of these persistent trains have now been observed and measured, and their altitudes have in many cases been determined by simultaneous observations at two or more stations. They average about ten miles in length. They are at first straight narrow streaks, but gradually expand to a width of a mile, and become curved and twisted into various shapes, as they are drifted by the wind. These transformations art) illustrated in the accompanying drawing. A shows the appearance of a train a few seconds after the nucleus of the meteor had disappeared. Soon after its appearance the train was sixteen to eighteen miles in length and at an altitude of fifty,six miles above the earth, as determined by triangulatton from Sidmouth and Cardiff, which are fifty miles apart. It appeared lancelike for a few moments and then was seen to be bending like a floating ribbon, and also expanding tn width, until it assumed the appearance shown at B. A meteor having a remarkably persistent train ap- PUPUl<l1' 'ciellce M ollthly. Meteor train seen at Sidmouth and Cardiff, England. Observed on November 14, 1866, at 1:08 A. M. Visible until 1:20 A.M. peared over the south of England at 7: 30 P. M., February 22nd, 1909. The train gradually increased in brilliancy, and twisted about, assuming grotesque shapes. A part of it drifted to the northwest at a velocity of 80 miles an hour, and remained plainly visible until 9:30 or 10 o'clock; i. e., over two hours. Another part drifted much faster; viz., at the rate of 300 miles an hour, according to the calculations of Mr. Denning, the well-known English authority on meteors. Prof. Trowbridge finds that the altitude above the earth',s surface at which persistent meteor trlins occur when seen at night is usually confined to definite limits between forty-five and sixty-five miles; though the path of the meteor may extend far above and below these limits. He terms this' stratum of the atmosphere the meteor train zone. . Be assumes therefore, that in this zone there .. are conditions favorable to hoth the formation and the persistence of the luminosity' of the trains, and he believes that the principal condition is the pressure of the atmosphere at that particular elevation. He explains the train as a phenomenon of luminescence, similar to the afterglow following an electric discharge in a vacuum tube. When a body is very hot an immense number of negatively charged corpuscles or ions are given forth from it. Air containing free ions becomes a conductor of electricity, hence we have in a meteor rushing through the atmosphere a condition very like a long electrical discharge tube containing a gas at low pressure; the passage of the meteor forming a column of highly ionized air thirty or forty miles in length. Experiments with vacuum tUb's containing air of about the same density that is supposed to occur at the altitudes in qUestion, and'from 'which the oxygen has been mostly extracted, give the same slowly-fading afterglow, following an electric discharge. Thus, according to Prof. Trowbridge, it is not unlikely that the production of the light of a meteor train is connected directly with the highly ionized state of the air produced by the outpouring of electrons from the intensely heated meteor. The rapid lateral expansion of the train is explained as due mainly to gas diffusion, and the rate of such diffusion depends upon the pressure and the temperature of the gas. Accurate observations of the expansion of these tl'ains should therefore aid in determining the pressure and temperature of the atmosphere at the altitude at which the train occurs. Further the drift of the train furnishes a means of determining the direction and speed of the air currents at these great altitudes, which are far beyond: the range of meteorological balloons. Whatever may be' tllughL of ProL Trowbridge's explanation of the physical nature; of meteor trains, it is clear that their systematic observation should be considered a most important part of the general campaign of upper air research which is now engaging the earnest attention of meteorologists and astrophysicists, The Odor of the Rainbow TTi VERYBODY has heard of the pot of gold buried E at the end of the rainbow, but there is another old belief connected with this meteor that is not so familiar nowadays. The attention of meteorologists was called to it, a few years ago, by Mr. Richard Bentley, of the Royal Meteorological Society. It appears that over half a century ago a controversy took place in the English newspapers as to whether the rainbow emitted an odor. A belief in such an emanation existed in antiquity, and has been echoed by several modern poets. Thus it is mentioned in Pliny, Aristotle, and a Greek writer referred to by Coleridge, in his “Table Talk"; in the ;'Peripatetic Philosophy” of Georgius de Rhodes; in Bacon's “Sylva"; in Browne's “Britannia's Pastorals,” and more lately in a poem by Robert Snow. The origin of this curious belief is exphined by Mr. Bentley as follows: Everyone is familiar with the increase of scent given off by plants and shrubs on a warm evening after the air has been newly washed by rain. This would naturally often coincide with the appearance of a rainbow. A New Miniature Projection Apparatus Suitable for Legal P hotography T HE photographic work of experts in criminology is carried out usually with the aid of artificial light, at least when microscope objectives are used. In these cases the surface to be illuminated is usually only an inch or two wide; but the illumination must be intense and very uniform. The small projectors which are used by' physicians for the examination of the eye and throat are ill suited for this purpose. A writer in Die Umschau describes Geiger's new miniature projection apparatus, which may be used for photography as well as for other Ipurposes. As the accompanying illustration (Fig. 1) shows, the apparatus consists essentially of a stand carry'ng a resistance box and the lamp with its housing and projecting lenses. The lamp is a self-regulating, or fixed ppint, electric arc, which •consumes about 3 amperes of current, can be used on ordinary incandescent circuits and yields an exceedingly uniform light of about 300 eandle power. The two projecting lenses are mounted in a draw tube, by extending which more or less the diameter of the illuminated circle may be varied from an inch or two to twelve inches. Even with the largest circle the illumination is sufficient for photogm,phic purposes. The size of the cirde can also be limited by the use of diaphragms. The lamp ean be moved vertically and horizontally on the sland. and the upper part of the stand carrying the lamp, can be inclined at any angle by means of the joint and screw shown just below the resistance box. The projector is also provided with a mirror which can be turned and inclined in any direction.. By means of these various adjustments the diJensions and direction of the cone of light ean be ' adapted to any' purpose. This flexibility of the apparatus will be found especially valuable when the working space is limited. This apparatus will be found very useful for scientific photography in mamy fields. Here we will mention only its employment in photographing the so-called latent finger marks which are often used for the identification of criminals. Although these marks are called latent, they are not strictly invisible, but only difficultly visible, that is to say, a special kind of illumination is required to make them sufficiently conspicuous to be photographed. Various devices for producing such illumination are described in special treatises. Among these methods that of Stockis is the most successful, but it requires a special projec- Fig. 2. New miniature projection apparatus. tion lantern, the mounting and adjustment of which are troublesome and tedious. The writer has obtained excellent photographs of latent finger prints on glass with Geiger's miniature apparatus, arranged as shown in Fig. 2. A camera C with an objective of long focus and a correspondingly long bellows, is placed with its lens at the distance of twice its focal length from the piece of glass, gg, bearing the finger marks. The projector S is placed in an inclined position with its mirror almost in contact with the lens of the camera, so that the narrow conical pencil of light is nearly perpendicular to the surface of the glass and nearly coincident with the optical axis of the camera. With this arrangement the papillary ridges of the 21 Fig. 3. Photographs of finger prints obtained with the new miniature projection apparatus. fnger impression are intensely illuminated and appear on the positive photograph as white lines on a black ground. Figure 3 shows a negative impressn:n contained directly in the cfmera on bromide plper by' this method. This negative shows the papillary lines black on white ground, and is hence direetly comparable with the impression of the blackened finger on paper. The great simplicity of this method should lead speedily to its general employment in criminal practice.