PEITCHARD S WEDGE PHOTOMETER. 305 



Measurements of the Transmission of the Pritchard Wcdjc. 

 By S. p. LANGLEY. 



The recent discussions concerning the use of the Pritchard wedge may give 

 interest to the following description of a study of it made at the suggestion of 

 Professor E. C. Pickering, at Allegheny, by means of the bolometer, in July, 1886. 



The wedge experimented upon is graduated from to 6.6 inches, and we selected 

 for measurements of absorption the points 0.3, 1,8, 3.3 (middle), 4.8, and 6.3 inches 

 It is obvious that the increments of the thickness between these points are equal. 

 The measurements are made by sending a horizontal beam from the siderostat 

 through the position occupied by the wedge, w (Figs. 2 and 3), immediately behind 

 which is a micrometer slit, s (whose length is usually kept slightly less than the 

 height of the wedge), with doubly moving jaws set to two millimeters' aperture, 

 the direct beam first being allowed to pass through the slit before the wedge is 

 put in place ("wo ivedge"), and then the wedge being introduced and slid succes- 

 sively into the positions 0.3, 1.8, 3.3, etc. 



As all the measures are taken by means of the bolometer, it will be well to 

 precede them by some examples of the degree of accuracy obtainable by it. The 

 probable error of a single measurement by the bolometer on any constant source 

 of moderate radiant heat is but a fraction of one per cent. 



An absolutely constant source is unattainable, but we give as an illustration ten 

 consecutive readings made in connection with the following experiments, and with 

 the same bolometer (No. 1), on a slowly-cooling Leslie cube. The unit of deflec- 

 tion is a millimeter on the cylindrical scale of the galvanometer.* 



DEFLECTION. 



(Source of Heat, Leslie Cube.) 



356 

 355 

 854 

 355 

 355 

 354 

 354 

 353 

 353 

 352 



Mean 354.1 ± 0.3 



* This part of the apparatus is more particularly described in an article " On hitherto Unmeasured Wave- 

 Lengths," in the American Journal of Science for August, 1886. 



