This is viewed via the lens _G_ and two tiny points of light appear in the field of the ocular due respectively to reflection from the front and back of the mirror _B_, the latter slightly fainter than the former. Alongside or between these the real star image can be brought for a comparison, and by turning the polarizer through an angle α the images can be equalized with the real image. Then a similar comparison is made with a reference star. If A be the brightness of the former and B of the latter then A/B = sin²α/sin²ββ The Zöllner photometer was at first set up as an alt-azimuth instrument with a small objective and rotation in altitude about the axis _C_. Since the general use of electric lamps instead of the inconvenient flame it is often fitted to the eye end of an equatorial. Another very useful instrument is the modern wedge photometer, closely resembling the Zöllner in some respects but with a very different method of varying the light; devised by the late Professor E. C. Pickering. It is shown somewhat in diagram in Fig. 150. Here as before O is the eye end of the tube, B the plane parallel reflector, C the side tube, L the source of light D the diaphragm and A the lens forming the artificial star by projecting the hole in the diaphragm. In actual practice the diameter of such hole is 1/100 inch or less. [Illustration: FIG. 151.—Simple Polarizing Photometer.] The light varying device W is a “photographic wedge” set in a frame which is graduated on the edge and moved in front of the aperture by a rack and pinion at R. There are beside colored and shade glasses for use as occasion requires. The “photographic wedge” is merely a strip of fine grained photographic plate given an evenly graduated exposure from end to end, developed, and sealed under a cover glass. Its absorption is permanent, non-selective as to color, and it can be made to shade off from a barely perceptible density to any required opacity. Sometimes a wedge of neutral tinted glass is used in its stead. Before using such a “wedge photometer” the wedge must be accurately calibrated by observation of real or artificial stars of known difference in brightness. This is a task demanding much care and is well described, together with the whole instrument by Parkhurst (Ap. J. 13, 249). The great difficulty with all instruments of this general type is the formation of an artificial star the image of which shall very closely resemble the image of the real star in appearance and color.