OP ARTS AND SCIENCES. 231 



XIII. 



THE WEDGE PHOTOMETER. 

 By Edward C. Pickering. 



Presented May 10, 1882. 



Much attention has recently been directed to the use of a wedge of 

 shade glass as a means of measuring the light of the stars. While it 

 has been maintained by various writers that this device is not a new 

 one, the credit for its introduction as a practical method of stellar 

 photometry seems clearly to belong to Professor Pritchard, Director 

 of the University Observatory, Oxford. Various theoretical objec- 

 tions have been offered to this photometer, and numerous sources of 

 error suggested. Professor Pritchard has made the best possible reply 

 to these criticisms by measuring a number of stars, and showing that 

 his results agreed very closely with those obtained elsewhere by wholly 

 different methods. His instrument consists of a wedge of shade glass 

 of a neutral tint inserted in the field of view of the telescope, and 

 movable so that a star may be viewed through the thicker or thinner 

 portions at will. The exact position is indicated by means of a scale. 

 The light of different stars is measured by bringing them in turn to the 

 centre of the field, and moving the wedge from the thin towards the 

 thick end until the star disappears. The exact point of disappearance 

 is then read by the scale. The stars must always be kept in the same 

 part of tl^e field, or the readings will not be comparable. By a long 

 wedge the error from this source will be reduced. A second wedge 

 in the reversed position will render the absorption uniform throu"^hout 

 the field. Instead of keeping the star in the same place by means of 

 clockwork, the edges of the wedge may be placed parallel to the path 

 of the star, when the effect of its motion will be insensible. To 

 obtain the best results the work .^-hould be made purely differential, 

 that is, frequent measures should be made of stars in the vicinity 

 assumed as standards. Otherwise large errors may be committed, due 



