[Investigations on Light and Heat pulilishcd willi Appropriation from the Kumford Fund.] 



VI. 



PritcharcPs Wedge Photometer. 

 By S. p. LANGLEY, C. A. YOUNG, and E. C. PICKERING. 



Presented November 10, 1886. 



The attention of astronomers has been directed to the use of a wedge of shade 

 glass as a photometer by the pnbhcation of the Uranoinetria Oxoniensis. The 

 excellent measures contained in this work of the light of the stars were made with 

 an instrument devised by Professor Charles Pritchai'd. A sHding wedge of shade 

 glass was inserted between the eye of the observer and the eye lens of a telescope. 

 The star to be measured was brought into the field of view, and the wedge moved 

 until the star disappeared. A graduated scale attached to the wedge served to 

 measure its position, and consequently the thickness of glass required to render the 

 star invisible. In order that I might submit an instrument of this kind to a careful 

 test, I requested Professor Pritchard to order the construction for the Observatory 

 of Harvard College of a photometer like his, which he kindly consented to do. 

 On receiving the instrument, it seemed best to subject it to an examination at 

 various observatories. The neutral color of the glass is of course a matter of prime 

 importance. To the eye this particular wedge is all that can be desired, and prob- 

 ably the errors from this source would not affect ordinary star measurements. A 

 severer test is, however, required to detect small systematic errors, especially in 

 the case of colored stars. For this purpose it is necessary to determine the absorp- 

 tion of each portion of the wedge for rays of various wave-lengths. The bolometer 

 appears to be the best instrument for this purpose, and Professor Langley agreed to 

 make the required measurements, the result of which is given below. 



To determine how far the instrument is to be recommended to those undertaking 

 photometric observations, it was desirable that it should be tried by some one skilled 

 in the use of astronomical and physical instruments. It was also preferable that the 

 observations should be made by some one who had not acquired special skill with 

 any one form of stellar photometer, and thus become prejudiced in its favor. These 

 conditions are fulfilled by Professor Young, who fortunately was willing to undertake 

 the required examination, and who has kindly prepared the statement given below. 



