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PRITCHARD S WEDGE PHOTOMETER 



Some Photometric Observations with the Pritchard Wedge 



By C. A. YOUNG. 



At the request of Professor Pickering, I have made a few observations with the 

 Pritchard wedge photometer belonging to the Harvard College Observatory. I 

 regret that unfavorable weather and other circumstances have prevented the series 

 from being more complete. The instrument has the wedge next the eye. The* eye- 

 piece, which magnifies the star image, is a single lens, in the focus of which is a small 

 diaphragm. The lowest power lens of the three provided with the instrument was 

 always used, and the ^-incli diaphragm. With the 23-inch telescope the magnifying 

 power was about 300, and the field of view was about 2 J', far too small for con- 

 venience. Great difficulty was found in keeping the eye so placed as to receive the 

 emergent pencil, as the long, flat surface of the metal plate that protects the wedge 

 was in the way of the nose and forehead. If the wedge, instead of being next the 

 eye, were placed at the principal focus of the eye-piece, where micrometers are 

 usually put, it would be much easier to use the instrument ; nor do I believe that, 

 with proper precautions, there would be any loss of accuracy. There should be 

 also some device for automatically recording the readings without the necessity of 

 using a light. Being without an assistant available for the purpose, I had to make 

 and record my own readings, and, after making a reading and recording it, it was 



necessary to wait a considerable time until the eye had regained its sensitiveness 



before making a new reading. By taking the precaution to make the extinctions 



always with one eye (the left), and to keep that eye closed while making the reading 



and record with the other eye, I found it possible to reduce this waiting period 



somewhat, and to make and record the extinctions at the rate of about two in five 

 minutes. 



The observation of extinctions was found very trying to the eyesight, — the 

 intent gazing into absolute darkness after a luminous point that was almost invisible, 

 or visible only by intermittent glimpses, and the gradual pushing of the wedge until 

 one was sure that the star at last just could not be seen. On two out of the four 

 evenings the work was stopped at the end of about two hours by an attack of 

 transient hemiopsia. However it may be in respect to accuracy, it is unquestionable 

 that the observation of extinctions is much more wearisome and difficult than that 

 of equalizations, as in the various forms of double-image photometers. 



The observations were made upon six stars in the A. A. A. S. star magnitude 



