The meridian photometer was designed to avoid this small scope. The photometric device is substantially the same as in Fig. 148. The objects compared are brought into the field by two exactly similar objectives placed at a small angle so that the images, after passing the double image prism, are substantially in coincidence. In front of each of the objectives is a mirror. The instrument points in the east and west line and the mirrors are at 45° with its axis. One brings Polaris into the field, the other by a motion of rotation about the telescope axis can bring any object in or close to the meridian into the field alongside Polaris. The images are then compared precisely as in the preceding instance.[25] There are suitable adjustments for bringing the images into the positions required. [25] For full description and method see H. A. Vol. 14, also Miss Furness’ admirable “Introduction to the Study of Variable Stars,” p. 122, et seq. Some modifications are described in H. A. Vol. 23. These direct comparison photometers give results subject to some annoying small corrections, but a vast amount of valuable work has been done with them in the Harvard Photometry. The various forms of photometer using an artificial star as intermediary in the comparison of real stars differ chiefly in the method of varying the light in a determinate measure. Rather the best known is the Zöllner instrument shown in diagram in Fig. 149. Here _A_ is the eye end of the main telescope tube. Across it at an angle of 45° is thrown a piece of plane parallel glass _B_ which serves to reflect to the focus the beam from down the side tube, _C_, forming the artificial star. [Illustration: FIG. 149.—Zöllner Photometer Diagram.] At the end of this tube is a small hole or more often a diaphragm perforated with several very small holes any of which can be brought into the axis of the tube. Beyond at _D_, is the source of light, originally a lamp flame, now generally a small incandescent lamp, with a ground glass disc or surface uniformly to diffuse the light. Within the tube _C_ lie three Nicol prisms _n_, _n__{1}, _n__{2}. Of these _n_, is fixed with respect to the mirror B and forms the analyser, which _n__{1} and _n__{2} turn together forming the polarizing system. Between _n_1_ and _n_2_ is a quartz plate _e_ cut perpendicular to the crystal axis. The color of the light transmitted by such a plate in polarized light varies through a wide range. By turning the Nicol _n_2_ therefore, the color of the beam which forms the artificial star can be made to match the real star under examination, and then by turning the whole system _n_2_, _E_, _n_1_, reading the rotation on the divided circle at _F_, the real star can be matched in intensity by the artificial one. [Illustration: FIG. 150.—Wedge Photometer.]