7. _Absolute magnitude._ The apparent magnitude of a star is changed as the star changes its distance from the observer, the intensity increasing indirectly as the square of the distance of the star. In order to make the magnitudes of the stars comparable with each other it is convenient to reduce them to their value at a certain unit of distance. As such we choose one siriometer. The corresponding magnitude will be called the _absolute_ magnitude and is denoted by _M_.[4] We easily find from the table given in the preceding paragraph that the absolute magnitude of the sun, according to ZÖLLNER's value of _m_, amounts to +3.4, of the moon to +31.2. For Jupiter we find _M_ = +24.6, for Venus _M_ = +25.3. The other planets have approximately _M_ = +30. For the absolute magnitudes of those stars for which it has hitherto been possible to carry out a determination, we find a value of _M_ between -8 and +13. We shall give in the third chapter short tables of the absolutely brightest and faintest stars now known. 8. _Photographic magnitudes._ The magnitudes which have been mentioned in the preceding paragraphs all refer to observations taken with the eye, and are called _visual_ magnitudes. The total intensity of a star is, however, essentially dependent on the instrument used in measuring the intensity. Besides the eye, the astronomers use a photographic plate, bolometer, a photo-electric cell, and other instruments. The difference in the results obtained with these instruments is due to the circumstance that different parts of the radiation are taken into account. The usual photographic plates, which have their principal sensibility in the violet parts of the spectrum, give us the _photographic_ magnitudes of the stars. It is, however, to be remarked that these magnitudes may vary from one plate to another, according to the distributive function of the plate (compare L. M. 67). This variation, which has not yet been sufficiently studied, seems however to be rather inconsiderable, and must be neglected in the following. The photographic magnitude of a star will in these lectures be denoted by _m′_, corresponding to a visual magnitude _m_. In practical astronomy use is also made of plates which, as the result of a certain preparation (in colour baths or in other ways), have acquired a distributive function nearly corresponding to that of the eye, and especially have a maximum point at the same wave-lengths. Such magnitudes are called _photo-visual_ (compare the memoir of PARKHURST in A. J. 36 [1912]).