important for the photometry of coal-gas that the standard light employed should be as nearly as possible of the same colour as the light of the gas-flame. The light of the lamp has been brought to this colour by a small admission of air below the point of combustion. Twelve holes, each 3 mm. in diameter, have been drilled through the outer tube 15 mm. below the top. The draught of the chimney is sufficient to determine an entry of air through these holes, which, happening under fixed conditions, is constant in amount. A short cylindrical screen goes round the chimney, the bottom of which is 58 mm. above the surface of the burner; and both screen and chimney are surrounded by a pentagonal shade, four panels of which are filled with blue glass, while the fifth, which is turned towards the disc of the photometer, is filled half-way down with a metal plate, which, overlapping the inner screen, allows only the light from the lower part of the flame to fall on the disc of the photometer. By supporting the lamp at such a height that the bottom of the screen is level with the centre of the disc, errors of parallax are avoided. Mr. Sugg, I believe, observed, and Mr. Dibdin has proved, that with the flame of an argand burner the light thus given by the lower part of the flame is independent, within wide limits, of the total height of the flame. The observation is true, not only for a particular burner, but for many, and probably for most, argand burners. Many alterations have been made in the structure of this lamp, and it has generally been found, though not always with equal exactness, that the height of the flame did not affect the light emitted from its lower part. As the lamp is now arranged the height of the flame can be observed through the blue glass panels, and whether the top is just visible above the circular screen or rises to the top of the panel, the light which falls upon the disc of the photometer does not alter measurably. The adjustment of the light to the value of ten candles-in other words, the adjustment of the height of the circular screen-has been effected by a number of comparisons between the lamp and the one-candle pentane standard.

14. On the Cause of the Spurious Double Lines sometimes seen with Spectroscopes, and of the Slender Appendages which accompany them. By G. JOHNSTONE STONEY, M.A., D.Sc., F.R.S.

Spurious double lines are sometimes seen with the spectroscope. In order to observe the phenomenon it is not necessary to use the complete spectroscope, noi is monochromatic light essential. It is sufficient to look with the telescope of the instrument directly down the collimator, so as to see the image of the slit. If the instrument be now pointed towards, or nearly towards, a distant flame, and if the slit be narrowed down to a certain point, a spurious double line will be seen in the observing telescope, instead of a correct image of the slit. The phenomenon will be produced under the circumstances which most readily admit of investigation when the incident light is restricted to a single beam of plane waves, falling on the slit either normally or obliquely; and incident light which sufficiently approximates to being of this kind is easily provided by placing a coarse supplementary slit in front of the lamp flame and allowing only the light which passes through this slit to reach the collimator.

Under these circumstances it will be found that when the light falls normally on the slit of the spectroscope it will form an image in the observing telescope which, with a certain width of slit, becomes a rather coarse double line bordered on either side by an exceedingly fine hair-like appendage, which is visible only when the light is sufficiently intense. By causing the incident light to fall obliquely the two constituents of the double line may be made to thin down, leaving a considerable dark interval between them, and then present very much the appearance of the sodium lines when fine. In intermediate positions of the incident light the interval between the two constituents of the double line is occupied by a bright ruling of hair-like appendages, varying in number with the inclination of the incident light. The conditions of the experiment may be modified in other ways, and other appearances produced-notably a flare consisting of a ruling of bright lines fading out in one direction.

All these appearances belong to the same class as the spurious disk of a star with its attendant diffraction rings when viewed through a telescope, and as the images which very minute objects present when examined by the microscope. In all these cases the image, as was long ago pointed out by Fraunhofer, is not a reproduction of the object. In fact, when the object is so small or so remote that the rays of light that reach the collimating lens from different parts of it differ only by a part of a wave-length or by a few wave-lengths, the diffraction phenomena which are always present become of large size. These, in the cases we have to deal with, take the form of a direct beam of light spread out, with coloured fringes extending like a fan to the right and left. If the telescope with which the object is viewed can take in the whole of these we get a true delineation of the object. But if it can only take in some of them, then the image we see is not a representation of the object, but the image of another object, viz. of that object which would emit the same central beam and those only of the fringes which the instrument can grasp, but not any of the others. In all cases this apparent object differs from the real object, and in some cases is wholly unlike it.

If, for example, after removing the eye-piece, we look directly at the light transmitted by the collimating lens, and make the adjustments such that the instrument takes in only the central beam and the first diffraction spectrum on either side, then on replacing the eye-piece we shall see a double line instead of a correct image of the slit. This, with some very thin appendages which can only be seen when the light is sufficiently bright, is, in fact, the kind of object which would emit a central beam and a first diffraction spectrum, and these only, of those breadths, in those phases, and with that ratio of the brightness of the central beam to the first diffraction spectrum which prevail between those parts of the light as actually received by the collimating lens.

The image that presents itself might conceivably be calculated by dividing the wave front as it passes the collimating lens into pairs of strips, and integrating the illumination these produce on the focal plane. This is the method employed in the case of the spurious disks of stars. But even if this method were practicable it would be inferior to a tentative geometrical treatment of the problem which, though it does not furnish the exact result, is more instructive. The geometrical method makes clear, which the integrations would not, why the thin appendage lines present themselves in addition to the main double line.

It was obviously desirable to attack the problem in its simplest form, which occurs when the incident light reaches the slit of the collimator normally. It is possible from general considerations, based on the theory of diffraction gratings, to conclude that then the main part of the light of the image as seen must be concentrated into two parallel strips constituting a so-called double line. The next step is to assume a distribution of light in two such strips which would produce, and which therefore would be the image formed by, a central beam and diffraction spectra of which those of even orders, the 2nd, 4th, &c., are of zero illumination. This can be done in various ways, producing different distributions of the light between the central beam and the spectra of odd orders. The next step is to select among these one which gives nearly the same ratio of brightness of the first spectrum to the central beam as prevails in the light which actually reaches the collimator from the slit. This was effected by making the strips broad, brighter in the middle, and with an interval between them about one-seventh of their width. The next step is to modify this distribution of light, so as to secure zero illumination in the third spectrum, while retaining the extinction of the spectra of even orders. This was effected by removing one-eighth of the light from each of the two strips; transferring half of this eighth to a certain part of the other strip, and with the other half forming an appendage line outside the strip, at a distance from it of nearly half its width, and only one-seventh as broad. The distribution of light, when modified in this way, on both the constituents of the double line would present the appearance of a somewhat coarse double line, with a narrow interval between them, and accompanied by two hair-like appendage lines, one to the right and the other to the left of the main double line; and this resembles the image as it actually presents itself in the telescope of the instrument.

We thus see that part of the light must appear in the form of faint appendages

in order to reconcile the extinction of the third spectrum with the simultaneous disappearance of the spectra of even orders. To get rid of the distant and fainter 5th, 7th, &c., spectra consistently with the other conditions of the problem in general requires a further modification of the main lines, a modification of the appendages already existing, and the introduction of other still fainter appendages. It should be mentioned that it is possible to make the adjustments such that the appendage lines, or some of them, shall overlie the main lines, and become merged in them, so as not in the observations to be distinguishable from them.

Having, by the geometrical treatment of some simple cases, made ourselves acquainted with the general progress of events, it is best to study by observation what the details become with each of the innumerable distributions of light that can be made to pass the collimating lens, and to record the principal results; for it would be an endless task to attempt the prediction of every phase of so Protean a phenomenon. The observations are of a simple kind, since the image which is formed in each case may be directly viewed in the telescope, and we can also easily ascertain what has been the light that has produced this image, by simply taking out the eye-piece and then looking down the tube of the telescope and through its object-glass. We thus see the collimating lens, and those parts of the central band and diffraction fringes formed by the slit which have reached the collimating lens, sc that these become known.

15. On the Luminosity observed when a Vacuum Bulb is broken.
By JOHN BURke.

It was noticed by Beccaria that a luminous effect was produced when vacuum bulbs were broken in the dark. After making some experiments upon the subject he attributed the luminous appearance, not to the breaking of the glass, but to the dashing of the external air, on the inside, when it was broken. Professor J. J. Thomson, in his work on 'Recent Researches in Electricity and Magnetism,' has brought forward the phenomenon observed by Beccaria as likely to confirm Mr. Crookes's theory of phosphorescence in a vacuum tube. The two phenomena, however, that of phosphorescence in a vacuum tube and that produced by the breaking of vacuum bulbs, seem to be totally distinct; and the luminosity in the latter case seems to be due to the collisions of the particles of glass with each other. Beccaria also obtained a light when air was allowed to strike against bodies placed inside a receiver, by the bursting of a bladder, but this was undoubtedly due to the burning portions of the bladder, that were stopped in their descent by the articles within the receiver, which he appears not to have observed.

The following experiments tend to disprove Beccaria's theory. A number of fragments of broken glass were supported at the mouth of the receiver of an airpump by an arrangement such that when air was allowed to rush into the receiver by the removal of a thick glass plate which served to cover the opening at the top, the pieces of glass were capable of descending with the air. The same luminous effect was obtained as when vacuum bulbs were broken.

A single spark was visible when only one piece of glass was employed, and appeared as though it had been caused by the striking of this against the side of the receiver. When no fragments of glass were employed no light was observed, even when a number of articles were placed at the bottom of the receiver.

Air was made to issue forth, on the surface of various substances, and especially on the sharp edges of broken glass, from bottles containing the air in a highly compressed state, but without any luminous effect.

The same results were obtained with other gases besides air.

A light was obtained when two large pieces of glass were violently struck, but no light was observed by the mere breaking of glass either in air or in a vacuum. Other substances instead of glass were also employed, such as cast iron, steel, copper, ebonite, sealing-wax, bone, but without any luminous effect.

All the experiments made seem to point to the conclusion that the phenomenon is not due to the violent impact of the air on the glass but to the collisions of the fragments of glass with each other.

16. On the Correction of Optical Instruments for Individual Eyes.

By TEMPEST ANDERSON, M.D., B.Sc.

The subject of astigmatism is now well understood, and the public are beginning to recognise that a very large proportion of cases of defective sight require a cylindrical as well as a spherical element in the corrective spectacles, and can by this means obtain a much greater acuteness of vision than without it. It is also well known that ordinary myopic or hypermetropic eyes, even if presbyopic, can obtain perfect vision with a telescope or microscope by adjusting the focus; but it is quite different with astigmatic eyes, for as the image comes into focus with one meridian of the eye, it goes out with the other.

Persons with such eyes when ordered spectacles are often rather casually told that they must wear their glasses when using a telescope or microscope, but there the matter usually ends, for the spectacle glass is found to come against the eyepiece, and even if it does not, the spectacle lens is not centred on the instrument, and various ghosts and false images are formed, which cause the glasses to be laid aside.

Few eyes are altogether free from astigmatism; probably half could have their vision perceptibly improved by suitable cylindrical lenses; and though the defect is not sufficient to make it worth while to wear spectacles, it is greater than would be allowed to exist in a telescope or microscope by a good maker. The defect is, of course, more noticeable in the use of instruments which give a pencil of rays which fully fills the pupil, such as the lower powers of telescopes and opera glasses and hand-magnifiers.

The remedy is simple, viz., to mount a cylindrical lens of power suitable for the individual eye in a cap, which can be applied to the eye-piece of the instrument. The lens can be properly centred, and causes no ghosts or false images, and being close to the lens of the eye-piece takes up very little room, and allows the eye to come convenienly near the instrument. Each observer will have one lens to suit his own eyes, applied to his own instrument; but for the benefit of those who like a complete instrument with adjustments to suit their friends, I may point out that it is not necessary to correct the spherical error of the eye, as this can be done by focussing, so that a set of caps containing, say, a dozen cylindrical lenses of the ordinary powers would enable a telescope to be corrected for almost any observer, provided he knew what cylinder he required.

17. How the Misuse of the Word Force,' in Attractions, Electricity, and Magnetism, may be avoided without much departure from existing practice. By Dr. G. JOHNSTONE STONEY, F.R.S.

All the classical writers on dynamics of the early part of the present centuryLaplace, Lagrange, Poisson, Gauss, and many others-used the word Force in two distinct senses, viz., for F the 'Moving force,' and for f the Accelerating force.' A distinct improvement was introduced several years ago when the word force was restricted to the former of these meanings, and when the 'accelerating force' was named the acceleration, or, as it ought rather to be called, the accelerator.

There is, however, one exception. In treatises on Attractions it is still usual to call f the force of attraction. There seems no sufficient excuse for this; and it is therefore suggested that the word accelerator be used instead. We may then say in dealing, for instance, with gravitation

Next as regards Potential. This term is used in Dynamics, Electricity, and Magnetism to designate physical quantities of different dimensions; but in all it is that factor of energy which, in order to convert it into energy, requires to be multiplied

In Dynamics, by a mass.

In Electricity, by a quantity of electricity.
In Magnetism, by a quantity of magnetism.

This may be stated conveniently by saying that

In Dynamics, Potential is the dynamic energy-factor. In Electricity, Potential is the electric energy-factor. In Magnetism, Potential is the magnetic energy-factor. Now a similar treatment of the term accelerator would at once carry out consistency in our nomenclature, would point attention to the true relation between these three sciences, and would free us from one part of the misuse of the word force which is now prevalent. If this suggestion be adopted, we shall speak of

The dynamic accelerator, viz., that force-factor which requires to be multiplied by mass to convert it into a force;

The electric accelerator, viz., that force-factor which requires to be multiplied by a quantity of electricity to convert it into a force; and

The magnetic accelerator, viz., that force-factor which requires to be multiplied by a quantity of magnetism to convert it into a force.

And these terms will take the place of what are now miscalled the Electric Force in ar Electric Field, and the Magnetic Force in a Magnetic Field; physical quantities which are in reality not forces at all, but force-factors.

There will then remain only one, but it is the greatest offender of them all, viz., Electro-motive Force-an abominable term, since it is not a force, nor even a forcefactor. It is in reality an energy-factor. Why not substitute the term Potency or Potential Range, which would exactly describe what it is, and which would offer the additional convenience of enabling us to speak, not only of the Potency of a battery or dynamo, extending over the entire current from pole to pole, but also of the Potential Range of each piece of apparatus introduced into the circuit extending from the place where the current enters it to the place where it emerges? It is often convenient to be able to speak of this concisely and without periphrasis. The phrase potency of a battery seems the most appropriate one for indicating how much work the battery compels each unit of electricity that traverses the circuit to do. This is what the term ought to imply; and the term Potency does so without introducing a false analogy like the term Pressure which is sometimes used.

18. On a Nomenclature for very much Facilitating the Use of Systematic Measures. By G. JOHNSTONE STONEY, M.A., D.Sc., F.R.S.

In this communication the author recommended certain prefixes and affixes, which he had been engaged for several years in testing, and which provide a means of avoiding periphrases and of speaking and thinking concisely about matters with which modern science has frequently to deal. The syllables suggested are

Hyper-, -ein, -et, -o-, -el, and -ane.

The metric system of weights and measures furnishes a complete decimal series of lengths and masses, and of the physical quantities derivable from them, such as volume, density, &c.: i.e., it presents us not only with the metre, the gram, the cubic metre, and so on, but also with all the decimal multiples and submultiples of each of these. Moreover, it only needs to use the metric system with the second as our unit of time to have an equally complete series of every other physical quantity, such as density, velocity, force, and energy in dynamics, along with the various other physical magnitudes with which the sciences of electricity and magnetism are separately concerned.