The second class of physical photometer is that depending on the change (usually increase) of electrical conductivity of the element selenium when illuminated. A sensitive selenium bridge (frequently termed selenium cell) may be made by winding four strands of fine nickel or platinum wire round a sheet of some good insulator which has been covered on one side with a thin layer of purified amorphous selenium. If two alternate strands of wire are then removed, the remaining strands are separated by a long strip of selenium of the same breadth as the diameter of the wire. By heating the bridge to a temperature of about 180° C for five minutes or longer the selenium is changed to the metallic form and the conductivity of the path between the wire then varies with the illumination of the bridge. Many other forms of cell have been developed for various purposes. This type of physical photometer is not only subject to the disadvantage that its luminosity curve is different from that of the eye and is, more over, not the same at all intensities of illumination, but it has the additional defect of a lag both in response to stimulation and in recovery after the illumination has been cut off.
Various methods may be employed for carrying out a measure ment with such a *cell-filter combination. The simplest is to have the combination illuminated in succession by the sub-standard and the lamps to be measured, and to balance the photoelectric current in each case by that from a simple photoelectric cell illuminated by a comparison lamp mounted on a bench (see fig. 12). The two cells, with their batteries, are connected in a Wheatstone bridge arrangement with a sensitive electrometer, and the distance of the compari son lamp from the cell it illum inates can be altered until the electrometer ceases to show a deflection. The illumination of the cell-filter combination is then proportional to pro vided both cells follow the same law connecting photoelectric current and illumination. It is unsafe to assume that this is the case over a very extended range of illumination, so that the sub-standard and the test lamps must be arranged to give illuminations of the same order at the cell filter combination. In the case of a measurement of luminous intensity this can be done quite easily, but when the cell is being used in combination with a sphere or cube for the measurement of luminous flux, some auxiliary means must be adopted for adjusting the value of the illumination. A sector disc may be used since Talbot's law is rigorously obeyed by a cell, its re sponse to light being practically instantaneous. For the i sensitivity of a photoelectric cell, a three-electrode valve is often employed. Such a combination will, in conjunction with a suitable galvanometer and chronograph, enable records of a varying illumination (e.g., daylight) to be obtained.
The earliest records we possess as to the positions of the stars have always been accompanied by estimations of their relative brightness; the stars being arranged in "magnitudes" according to their apparent luminosities. Thus, in the catalogue of stars published by Ptolemy (c. A.D. but which had probably been formed 30o years previously by Hipparchus, the stars visible to the unaided eye were divided into six classes or magnitudes, the brightest stars being designated "first magnitude stars" and those just visible to the naked eye forming the "sixth magnitude." Each
class was further sub-divided into three sub-groups. This process of arranging the stars by eye estimates (with the unaided eye at first and subsequently with the aid of telescopes) was further developed by Flamsteed and the Herschels, and culminated in the work of Argelander, Schonfeld and Kruger, who published three catalogues (1859-62) of celestial co-ordinates and magnitudes known as the Bonn Durchmusterung. This catalogue included the enormous number of 324,188 stars. An additional volume con taining 133,659 stars south of the equator was published in 1886 and the work was further extended in the southern hemisphere by B. A. Gould, J. M. Thorne and others. In the Bonn Durchmuster ung, or "B.D.," each magnitude is sub-divided into ten divisions, a decimal notation being used.
So far no attempt was made to define the quantitative relation ship between different magnitudes. In modern times a definition due to Pogson has arisen, and we define a difference of one magni tude between two stars as meaning that the light that reaches us from the first star is k times the light which reaches us from the second. The constant k is chosen so as to make a difference of five magnitudes correspond to a light ratio of 1 oo and this means that we must have =0.4. We cannot observe all the radiation emitted by a star. If the instrument used for registering the light be the human eye, it will not respond to radiation either in the extreme ultra-violet end of the spectrum or in the extreme infra red. Now if two stars are of different colours, the proportion of the whole radiation represented by the light which affects the eye will be different for the two stars, and the adoption of the above definition as it stands would lead to difficulties. We can retain it, however, for stars of the same colour and then, as far as visual magnitudes are concerned, we define two stars of different colour as being of the same visual magnitude when they appear equally bright to the eye. With the introduction of photo graphic methods, a photographic scale has arisen defined in the same way; two stars being of equal photographic magnitude when they produce equal photographic effects. Reference will be made below to certain difficulties arising out of these definitions. With regard to the zeros or starting points of these visual and photo graphic scales, the zero of the measured scale of visual magni tudes has been fixed so that the magnitudes agree approximately with those in the "B.D." for the mean of stars down to the sixth magnitude, whilst the zero of the photographic scale is defined so that for the white stars (of spectral type Ao) of the sixth magni tude (between visual magnitudes 5.5 and 6.5) the visual and photo graphic magnitudes are equal.
The actual methods of procedure, both visual and photographic, will be given in outline later in this article. In recent years an enormous stimulus has been given to stellar photometry by the realization that it can lead us to good estimates of the distances of remote objects. It was pointed out by Hertzsprung (see STAR) , that the period of certain variable stars was related to their abso lute magnitude, i.e., to the magnitude that they would appear to have when viewed by an observer at a certain standard distance. By observing the period the absolute magnitude can be deduced, and then, provided the apparent magnitude has been measured. the distance of the star can be inferred. The discovery and eiaoo ration of methods such as these has led to the extension of the photometric scale down to the loth magnitude. The scale of magnitudes defined numerically can of course be extended on the negative side, and, in point of fact, the visual magnitude of Sirius is —1.4 whilst that of the sun is —26.7.