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 are well adapted to solution by least squares or any equivalent device.

Photography of Nebulae and Clusters.—Some of the earliest and most striking successes in celestial photography were the pictures of nebulae. Dr A. A. Common (1841–1903), F.R.S., of Ealing, led the way in 1883 with a successful picture of the great nebula in Orion, taken with a 3 ft. concave mirror by Calver. Dr Isaac Roberts (1829–1904) was the first to show the real structure of the great nebula in Andromeda, by a photograph also taken with a reflector. In the clear atmosphere of the Lick Observatory in California, small nebulae were photographed in great numbers by Professor J. E. Keeler (1857–1900): and it was shown what a large percentage were spiral in form. Prof G. W. Ritchey, at the Yerkes Observatory, has followed up these successes with a 2-ft. reflector, and is constructing a 5-ft., to be erected on Mt Wilson (Cal.); but he has also shown that pictures of clusters are best taken with a telescope of long focus, such as the great Yerkes refractor; and incidentally that this telescope, although intended for visual work, can be adapted to photography by using a “colour screen” just in front of the plate, which sifts out the rays not brought to focus.

Photography of the Moon.—G. W. Ritchey has used the same device of a colour screen for the moon, and obtained even better pictures than those obtained at Paris, which were previously the best. The positions of a large number of craters and other points have been measured by Dr J. H. G. Franz and S. A. Saunder on photographs, and a new epoch in lunar topography has thereby been created.

Photography of the Planets.—Some striking successes have been obtained at the Lowell Observatory, Flagstaff, Arizona: by cutting down the aperture of the object-glass some of the delicate markings, called canals, on the planet Mars have been photographed, but even these do not approach what can be seen by the eye.

Photography of Comets.—Some wonderful pictures have been obtained of comets by Professor E. E. Barnard and others. Here, as in the case of nebulae, the photograph is superior to the eye in detecting faint luminosity, and delicate details of the tail structure have been photographed which could never be seen. In several pictures the tails have an appearance of violent shattering, and if successive pictures can be obtained at such times we may learn something of the nature of such disturbances.

Solar Photography.—The light of the sun is so intense that the chief difficulty is to obtain a short enough exposure. When successfully taken, photographs of the surface show the well known spots and the mottling of the surface. The image sensibly falls off in intensity towards the limb, owing to the absorption of light by the solar atmosphere; and the bright faculae (which are thus inferred to lie above the main absorbing layer) are seen near the limb. But an immense advance in solar photography was made about a dozen years ago by the invention of the spectroheliograph, which is an instrument for photographing in the light of one very definite colour—say a single hydrogen line. The faculous appearances can be photographed with this instrument all over the sun’s disk, instead of merely near the limb. The appearance presented varies enormously with the line selected, or (in the case of the wide “lines” in the spectrum, such as the H and K lines) with the particular part of the same line selected. But for a full account of such matters reference must be made to the articles and.

 PHOTOMETRY (from Gr. , light,  , a measure), the art and science of comparing the intensities or illuminating powers of two or more sources of light. As in all scientific

measurements, its methods are attempts to give quantitative accuracy to the crude comparisons made by the eye itself. The necessity for this accuracy in practical affairs of life has arisen because of the great development of artificial lighting in recent times. The eye soon learns to associate with any particular source of light a quality of brightness or power of illumination which diminishes with increase of distance of the source from the eye or from the surface illuminated. This quality depends upon an intrinsic property of the source of light itself, generally known as its “candle power.” The aim of photometry is to measure this candle power; and whatever be the experimental means adopted the eye must in all cases be the final judge.

In the photometric comparison of artificial lights, which frequently vary both in size and colour, direct observation of the sources themselves does not yield satisfactory results. It is found to be much better to compare the illuminations produced on dead white surfaces from which no regular reflection takes place, or through colourless translucent material uniformly illuminated by the light placed on the further side. By such processes there is always loss of light, and we must be certain that the various coloured constituents of the light are reduced in the same proportion. This necessary condition is practically satisfied by the use of white diffusing screens.

Two principles of radiation underlie many photometric applications, namely, the inverse square distance law, and J. H. Lambert’s “cosine law.” Both can be established on theoretical grounds, certain conditions being fulfilled. But as these conditions are never absolutely satisfied, the applicability of the two laws

must in the end be tested by experiment. Since we find that within the errors of observation four candles, placed together at a distance of 2 ft. from a diffusing screen, produce the same illumination as one candle at a distance of 1 ft., we may regard the inverse square distance law as satisfied. Thus if two lights of intensities A and B produce equal illuminations on a screen when their distances from the screen are respectively a and b, we at once write down the relation between the two intensities in the form A&#x202f;:&#x202f;B＝a2&#x202f;:&#x202f;b2. The theoretical basis of the law follows at once from the universally accepted view that light is energy radiating outwards in all directions from the source. If we assume that there is no loss of energy in the transmitting medium, then the whole amount of radiant energy passing in one second across any closed surface completely surrounding the source of light must be the same whatever the size or form of the surface. Imagine for simplicity a point source of light, or its equivalent, a uniformly radiating spherical surface with the point at its centre, and draw round this point a spherical surface of unit radius. Across this surface there will pass a definite amount of radiant energy, in other words a definite total luminous flux, E, which will be the same for all concentric spherical surfaces. Since the area of a spherical surface of radius r is 4&thinsp;&thinsp;r2, the flux which crosses unit area is E/4&thinsp;&thinsp;r2. This quantity is the “illumination.” It is measured in terms of the unit called the lux, which is defined as the illumination produced by a light of unit intensity on a perfectly white surface at a distance of 1 ft. In the great majority of photometers the illuminations are compared, and the intensities are deduced by applying the law of the squared distances.

Lambert’s cosine law has to do with the way in which a luminous surface sends off its radiations in various directions. It is a matter of common observation that the disk of the sun appears equally bright all over the surface. Careful measurements show that this is not strictly true; but it is sufficiently near the truth

would hold accurately. Again, when a glowing surface is viewed through a small hole in an opaque plate, the brightness is very approximately independent of the angular position of the incandescent surface. This is the same phenomenon as the first mentioned, and shows that the more oblique, and therefore larger, element of surface sends the same amount of radiation through the hole. Hence the amount per unit surface sent off 
 * to suggest that under certain definable conditions the law