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Rh measurement of as many rays as can be conveniently combined. One of the best methods of effecting this is by means of C. P. Matthews's integrating photometer. By the use of twelve mirrors arranged in a semicircle whose diameter coincides with the axis of the lamp, twelve rays are caught and reflected outward to a second set of twelve mirrors which throw the rays on to the surface of a photometric screen. This combined effect is balanced by the illumination produced by a standard lamp on the other side of the screen (see Trans. Amer. Inst. Elect. Eng., 1902, vol. xix.).

So long as the lights to be compared are of the same or nearly the same tint. the photometric match obtained by different observers is practically the same. If, however, they are of distinctly different colours, not only do different observers obtain different results but those obtained by the same observer at different times are not always in agreement. Helmholtz was of opinion that photometric comparison of the intensities of different coloured lights possessed no real intrinsic value. There can be little doubt that in a rigorous sense this is true. Nevertheless it is possible under certain conditions to effect a comparison which has some practical value. For example, when the intensities of two differently coloured lights differ considerably there is no difficulty in judging which is the stronger. By making the one light pass through a fairly large range of brightness we may easily assign limits outside which the intensities are undoubtedly different. After some experience these limits get close; and many experimenters find it possible, by taking proper precautions, not only to effect a match, but to effect practically the same match time after time. According to Abney, whose memoirs on colour photometry (Phil. Trans., 1886, 1892) form a most important contribution to the subject, the observer in making his judgment as to the equality of luminosity of two patches of colour placed side by side must not begin to think about it, but must let the eye act as unconsciously as possible. His method was to compare the coloured patch with white light given by a particular standard and cut down to the proper intensity by use of a Talbot's rotating sector, which could be adjusted by means of a suitable mechanism while it was rotating.

At the same time, although the eye may be able to effect a definite matching of two patches of colour of a particular luminosity, it has been long known that a change in the luminosity will destroy the apparent equality. This depends upon a physiological property of the retina discovered by J. E. Purkinje in 1825 (see below, Celestial Photometry). In virtue of this property the blue and violet end of the spectrum is more stimulating to the eye than the red end when the general luminosity is low, whereas at high luminosities the red gains relatively in brightness until it becomes more stimulating than the blue. Unless therefore account is taken in some definite measurable manner of the absolute brightness, there must always be some uncertainty in the photometric comparison of the intensities of differently coloured sources of light.

Instead, however, of trying to effect a photometric match in any of the ways which have been found sufficient when the sources are of the same or nearly the same tint, we may effect important practical comparisons in what is called hetero chromatic photometry by an appeal to other physiological properties of the eye. For example, the power of clearly discriminating patterns in differently coloured lights of various intensities is obviously of great practical importance; and this power of detailed discrimination may be made the basis of a method of photometry. According to this method two lights are arranged so as to illuminate two exactly similar patterns of lines drawn, for example, on the sides of a Ritchie wedge, and their distances are adjusted until the patterns are seen equally distinct on the two sides. Application of the usual distance law will then give the relation between the two lights. A discrimination photometer constructed on this principle has been designed by J. A. Fleming. Its results do not agree with the indications of an ordinary

luminosity photometer; for it is found that the eye can discriminate detail better with yellow than with blue light of the same apparent luminous intensity.

Another and very promising method of photometry depends upon the duration of luminous impressions on the retina. J. A. F. Plateau observed in 1829 that the blending into homogeneous impression of a pattern of alternate sectors of black and some other colour marked on a disk when that disk was rotated occurred for rates of rotation which depended on the colour used. A form of experiment suggested in Professor O. N. Rood’s Modern Chromatics seems to have been first carried out by E. L. Nichols (Amer. Journ. of Science, 1881). A black disk with four narrow open sectors was rotated in front of the slit of a spectroscope. When the rotation was not too quick the yellow part of the spectrum appeared as a succession of flashes of light separated by intervals of darkness of appreciable length, whereas towards both the red and violet ends no apparent interruption in the steady luminosity could be observed. As the rate of rotation increased the part of the spectrum in which flickering appeared contracted to a smaller length extending on each side of the yellow, and finally with sufficiently rapid alternation the yellow itself became steady This seems to show that the retinal image persists for a shorter time with yellow light than with light of any other colour; for with it the intervals of darkness must be shorter before 8. continuous impression can be obtained. Now yellow is the most luminous part of the spectrum as it affects the normal human eye; and E. S. Ferry (Amer. Journ. of Science, 1892) has shown that the duration of luminous impression is mostly, if not entirely, determined by the luminosity of the ray. Hence the determination of the minimum rate of intermittence at which a particular colour of light becomes continuous may be regarded as a measure of the luminosity, the slower rate corresponding to the lower luminosity. Although in the experiment just described the red part of the ordinary solar spectrum becomes continuous for a slower rate of intermittence than the yellow part, yet we have simply to make a red ray as luminous as the yellow ray to find that they become continuous for the same rate of intermittence. It is, however, highly improbable that the duration of impression depends only on the luminosity of the light and not to some extent upon the wave-length. There are indeed phenomena which require for their explanation the assumption that the duration of luminous impression does depend on the colour as well as on the brightness.

Nevertheless the luminosity is by far the more important factor, as shown by Ogden N. Rood’s experiments. He found (Amer. Journ. of Science, 1893) that, when a disk whose halves differ in tint but not in luminosity is rotated rather slowly, the eye of the observer sees no flickering such as is at once apparent when the halves differ slightly in luminosity. Rood himself suggested various forms of photometer based on this principle. In his latest form (Amer. Journ. of Science, Sept. 1899) the differently coloured beams of light which are to be compared photometrically are made to illuminate the two surfaces of a Ritchie wedge set facing the eye. Between the wedge and the eye is placed a cylindrical concave lens, which can be set in oscillation by means of a motor in such a way that first the one illuminated surface of the wedge and then the other is presented to the eye in sufficiently rapid alternation. The one source of light is kept fixed, while the other is moved about until the sensation of flicker disappears. From work with this form of instrument Rood concluded that “the accuracy attainable with the flicker photometer, as at present constructed, and using light of different colours almost spectral in hue, is about the same as with ordinary photometers using plain white light, or light of exactly the same colour.”

Various modifications of Rood’s forms have been constructed from time to time by different experimenters. The Simmance and Abady flicker photometer is an ingenious and yet mechanically simple method by which (as it were) the wedge itself is made to oscillate so as to throw on the eye in rapid succession, first the one side and then the other. The rim of a wheel of white material is bevelled