MODERN WORK ON VISUAL MAGNITUDES The bulk of observational work on visual magnitudes has been carried out at Harvard, and at the Harvard station in the southern hemisphere (but a considerable amount has been accomplished at Potsdam), and the standard catalogue of visual magnitudes for the brighter stars is the "Revised Harvard Photometry" (Har vard Annals, vol. 5o). This catalogue contains the computed mag nitudes of 9,110 stars, of magnitude 6.5 and brighter, distributed over the whole sky. A further catalogue (Harvard Annals, vol. 54) gives the magnitudes of 36,682 stars fainter than 6.5. The in strument used at Harvard was the meridian photometer designed by the late Prof. Pickering. The principle underlying the instru ment is the presentation of two stars in the field of view, the light from one being varied by a Nicol prism until the two stars appear of equal brightness. The ratio in which the incident light has been varied is read off from the angle through which the Nicol has been rotated, and the difference of magnitudes of the two stars thus obtained. The stars were all compared with a standard star, which was the Pole star in the case of the brighter stars and X Ursae Minoris for the fainter ones.
The photometer (Harvard Annals, vols. 14 and 23) consists of two telescopes placed side by side pointing due east, the light from the stars on the meridian being reflected into them by two mirrors inclined at an angle of 45° to this direction. If there were a star exactly at the Pole, one of these mirrors would be absolutely fixed and would constantly reflect the light of this star down the axis of its telescope; in practice a slight motion can be given to the mirror so as to keep in view the polar star selected as the standard of comparison. The second mirror (which projects a little beyond the first so as to obtain an unobstructed view of the meridian) can be rotated round the axis of the tele scope by means of a toothed-wheel gearing, and can thus be made to reflect any star on the meridian down the second telescope ; it is also provided with a small motion in the perpendicular direc tion, so as to command a degree or two on each side of the me ridian. Near the common eyepiece of the telescopes there is a double image prism which separates the light received from each into two pencils; the pencil of ordinary rays from one object-glass is made to coincide with that of extraordinary rays from the other, and the two remaining pencils are excluded by a stop. The two coincident pencils then pass through a Nicol prism to the eye of the observer, who by rotating the prism round its axis can equalize them at a definite reading depending on their relative intensities. This reading gives in fact the difference of magnitude between the two stars selected for comparison. It may be re marked that the position of the double image prism is important. It should be just within, not at, the common focus; this position prevents any noticeable colour in the images, and gives the ordi nary and extraordinary pencils a sufficient separation at the eye stop to permit the entire exclusion of one without the loss of any part of the other. If the prism were exactly at the focus, and any
part of the superfluous images were admitted, the resulting second ary images would coincide with the others and thus lead to errors in observing. As it is, owing to the construction of the instrument, if the secondary images appeared at all they would do so only as additional stars, near those under observation and too faint to produce any inconvenience.
Each observation consists of four comparisons ; after the first two the observer reverses the position of the star images in the field, and also reverses the double-image prism. The former pre caution is necessary to eliminate a curious error depending on the relative position of the images, which may amount to several tenths of a magnitude. Errors of this kind affect all estimates of the relative brightness of two stars in the same field, as has been repeatedly shown; a striking instance is given by A. W. Roberts of Lovedale, South Africa (Mon. Not. R.A.S., 1897), who found that his eye-estimations of the brightness of variable stars re quired a correction depending on the position-angle of the com parison star and ranging over nearly two magnitudes. In the Harvard work on the brighter stars, the magnitude of the pole star was provisionally assumed to be 2.0 at the pole. Each obser vation had to be corrected for the effect of atmospheric absorp tion, the coefficient for which was obtained from observations of circum-polars at upper and lower culminations. The provisional magnitudes thus obtained were finally corrected by a constant chosen so as to make the mean magnitude of the circum-polars equal to the mean magnitude of the same stars as given in the "B.D." Other types of photometer are Zollner's (Photometrische Un tersuchungen, h. 81), in which an artificial star is taken as the standard of comparison, and Pritchard's, in which the images of stars are viewed through a wedge and the wedge-reading at which the star is extinguished noted.
The comparison of different catalogues of visual magnitudes reveals small systematic differences which arise out of the diffi culties of estimating when two stars of different colour are equal in apparent intensity. The observation is complicated by what is known as the "Purkinje phenomenon." If a blue source of light and a red source appear equally bright to the eye, and if the intensity of each be diminished in the same ratio, they will no longer seem equally bright, the blue appearing the brighter. From this cause, magnitudes observed with meridian photometers of different aperture will differ slightly. It becomes obvious that visual magnitudes ought to be referred to some standard instru ment, and possibly to some standard observer, and it is difficulties such as these which are at the present time leading to the replace ment of visual magnitudes by what are known as photo-visual magnitudes.