Photographic Photometry

At Harvard the out-of-focus method has been used. In this case a series of exposures are made and for each exposure the plate is placed at a certain distance from the focus. Each star is now represented by a blackened patch on the plate, instead of by the usual image, and the density of the silver deposited in each patch is measured by comparison with a graduated wedge. In the case of the different exposures to the same star at different distances from the focus, the relative quantities of light falling upon unit area of the plate are determined from straightforward geometry, and the relation between the measured densities and the magnitude scale thus determined (Harvard Annals, vol. 59). It may be remarked that the out-of-focus method lends itself readily to the determination of the magnitudes of planets. For the purpose of determining the magnitudes of the sun and moon, a pin-hole camera was used by the Harvard observers, and they obtained —25.83 m. for the photographic magnitude of the sun and -II•20 m. for that of the full moon (Harvard Annals, vol.

59).

In recent years a great deal of care has been lavished upon the determination of the magnitudes of a sequence of stars extending down to the loth magnitude and situated at the North Pole of the sky. This having been done the magnitudes of stars in any other part of the northern sky can be obtained by photographing the field and the pole on the same plate and with the same ex posure. In order to eliminate atmospheric absorption the exposures should be made at the same altitude. Measures (say of diameter) can then be made of all the stars in each field, and the measures calibrated from the determined magnitudes of the Polar sequence. Several secondary sequences have been determined in this way in various parts of the sky and the work is being pushed into the southern hemisphere.

Colour-indices and Standardization.

The visual and photographic magnitudes are not necessarily the same. A red star, for example, may affect the eye strongly, but its effect on an ordinary photographic plate, which is chiefly sensitive to blue light, may be small. The difference between the photographic and visual magnitudes of a star (which in extreme cases may amount to II magnitude), reckoned in the sense photographic minus visual, is called its colour-index. However, a little con sideration will show that photographic magnitudes determined with different instruments (or different plates) will differ from each other. For example, in a refractor a large portion of the ultra-violet light is absorbed by the lens, and in a reflector this light is let through to a much greater extent. Clearly the photo

graphic magnitudes as determined with these types of instruments will be different. It is necessary to adopt some standard, and the International Astronomical Union, in defining the magnitudes of the standard polar sequence, has adopted the scale of the 6o-inch reflector at Mt. Wilson. The magnitudes m of stars in any cata logue can be connected with the standard magnitudes m', which are referred to the scale of the 6o-inch by an equation of the form m=m'+a+bC where C is the colour index and a and b are constants.

At Mt. Wilson Dr. Seares has devised a method of measuring colour-index directly. He photographs stars on colour-sensitive plates through blue and yellow filters and he defines the "exposure ratio" as the logarithmic ratio of the exposure times which produce blue and yellow images of the same size. The results have to be calibrated by comparison with colour-indices, but, this having been done, the method has the advantage of determining the colour directly instead of by differencing two separate magnitude determinations. A further direct measure of colour is provided by the "effective photographic wave-length" of a star. This may be defined as the wave-length of the light from the star which pro duces the maximum effect on a photographic plate. It is greater for the red stars than for the blue, and may be determined by placing a coarse diffraction grating over the aperture and then measuring the distance on the plate between the central image of a star and the most intense part of the lateral image produced by diffraction (Monthly Notices R.A.S., 82, 1921). Effective wave-lengths having been determined, they can be calibrated by comparison with colour-indices.

Photovisual Magnitudes.

Just as different photographic determinations differ, so will visual observers using different instru ments obtain different results. There is also the complication arising from the Purkinje effect. For this reason attempts have been made of recent years to replace visual determinations by photographic ones made on colour-sensitive plates through yellow screens. The resulting magnitudes are called photovisual magni tudes. They are roughly the same as the visual ones, at any rate for the brighter stars, and lend themselves readily to standardiza tion. Photovisual magnitudes have been determined at Mt. Wil son for the stars of the polar sequence. When compared with the Harvard visual results they show differences depending on colour, which become more pronounced for the fainter stars, and may amount to as much as three-tenths of a magnitude for red stars of the 12th magnitude.