1889.] 



A Compound Wedge Photometer. 



15 



IV. "A Compound Wedge Photometer." By E. J. Spitta, 

 L.RC. Phys. Lond. Communicated by Capt. W. de W. 

 Abney, C.B., F.E.S. Eeceived November 8, 1889. 



The idea of employing a wedge of neutral-tinted glass as a photo- 

 meter 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 £ Urano- 

 metria Nova Oxoniensis,' a catalogue of the relative brightness of the 

 brighter stars north of the equator. But the use of such an instru- 

 ment has always been limited hitherto to the comparison of the rela- 

 tive 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. Haying, how- 

 ever, 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. It is not within the 

 scope of this paper to give any detailed account of the many experi- 

 ments I have made with several wedges, but it is sufficient to say 

 that the wedge-form itself has been fully proved to be an important 

 factor in the production of the discordances to which reference has 

 been made, for the following reasons : — 



A point of light from its very definition implies that no sensible 

 portion of the wedge is occupied in its passage, but it requires very 

 little thought to perceive that when an area of sensible dimensions is 

 being dealt with this is by no means the case. Moreover, an ele- 

 mentary inquiry suffices to point out that if the area possess a 

 considerable diameter the light emanating from its lateral portions 

 will impinge on different thicknesses of the wedge, as shown to an 

 exaggerated degree in fig. 1, where AB is the transverse diameter of 



Fig. 1. 



the area, and CDEP the portion of the wedge employed. It is 

 evident if the differences in intensity be required between two disks 



