Stellar photometers may in principle be divided into three classes. (1) Those in which two actual stars are brought into the same field and compared by varying the light from one or both in a known degree. (2) Those which bring a real star into the field alongside an artificial star, and again bring the two to equality by a known variation, usually comparing two or more stars via the same artificial star; (3) those which measure the light of a star by a definite method of extinguishing it entirely or just to the verge of disappearance in a known progression. Of each class there are divers varieties. The type of the first class may be taken as the late Professor E. C. Pickering’s polarizing photometer. Its optical principle is shown in Fig. 148. Here the brightness of two neighboring objects is compared by polarizing at 90° apart the light received from each and reducing the resulting images to equality by an analyzing Nicol prism. The photometer is fully described, with, several other polarizing instruments, in H. A. Vol. II from which Fig. 148 is taken. A is a Nicol prism inserted in the ocular _B_, which revolves carrying with it a divided circle _C_ read against the index _D_. In the tube _E_ which fits the eye end of the telescope, is placed the double image quartz prism _F_ capable of sliding either way without rotation by pulling the cord _G_. With the objects to be compared in the same field, two images of each appear. By turning the analyzing Nicol the fainter image of the brighter can always be reduced to equality with the brighter image of the fainter, and the amount of rotation measures the required ratio of brightness.[24] This instrument works well for objects near enough to be in the same field of view. The distance between the images can be adjusted by sliding the prism _F_ back and forth, but the available range of view is limited to a small fraction of a degree in ordinary telescopes. [24] If A be the brightness of one object and B that of the other, α the reading of the index when one image disappears and β the reading when the two images are equal then A/B = tan²(α-β). There are four positions of the Nicol, 90° apart, for which equality can be established, and usually all are read and the mean taken. (H. A. II, 1.)