324 pritchard's wedge photometer. 



point marked zero, as found above, we have 7.78 for the reduction caused by 3.5 

 inches. This gives for the absorption per inch the ratio whose logarithm is 0.7, or 

 a somewhat greater value than that found by Professor Langley. The photograph 

 shows that the intensity of the spectrum transmitted by the wedge falls off rapidly 

 beyond A = 0.41, the photographic image becoming entirely invisible where X = 0.40. 

 The H and K lines cannot be detected in this spectrum, although readily seen in 

 the other images. The opacity of the wedge increases rapidly as the wave-length 

 diminishes, as shown by the bolometer. A second photograph was taken of the 

 light transmitted at the point marked 5.5 inches, and confirmed the above results. 

 The exposure lasted for nearly an entire day, and the brightest part was about 

 equal to a spectrum obtained in 10 seconds without the wedge. As before, the 

 II and K lines were invisible. 



In conclusion, the Uranometria Oxoniensis shows that valuable results may be 

 obtained with the wedge photometer in skilful hands. But the experiments described 

 above show sources of error which must be carefully studied before we can safely 



pply it to stars of differe 



catalogues. 



detecting small systematic errors in star 



