IV. A compound wedge photometer

Abstract

The idea of employing a wedge of neutral-tinted glass as a photometer has occurred to many observers—Dawes, Captain Abney, and others—and notably of late years to Professor Pritchard, of Oxford, who has produced with such an instrument his well known ‘Uranometria Nova Oxoniensis,’ a catalogue of the relative brightness of the brighter stars north of the equator. But the use of such an instrument has always been limited hitherto to the comparison of the relative intensities of such points of light as the stars present, its employment upon objects of sensible area being foreign to the ideas and requirements proposed in its construction. Having, however, attempted to use a photometer of this description upon disks of small but of various areas illuminated by a known amount of light, the discordances of the results forced upon me the necessity of modifying the construction of the photometer in a way which I believe will extend its sphere of usefulness.