446 Minchin — Reflection of Light by Colored Papers. 



l. 



O 



collimator T, in which the slit was adjusted to some desired 

 width. The second lamp was used at collimator T', first 

 directly in front of the slit and afterwards at one side, the 

 light from it being reflected into T ; by the reflecting surface. 

 9 Care was taken so to shield the lamps 



that no light reached the prism P, 

 except that coming through the slits. 

 To obtain a more uniform field-ground 

 glass plates were placed directly in 

 front of the slits. 



A revolving disk (fig. 2) similar to 

 that described by Brace* was placed 

 in front of T, by means of which the 

 intensity of the light admitted to the 

 slit could be varied by tenths, giving 

 0\L, 0*2, 0*3, etc., of the full intensity. 

 The disk was driven by an electric 

 motor as described by Murphy. \ 



With the lamps placed directly in 

 front of the two collimators and the 

 slit in collimator T set at a definite 

 width, the slit in T / was calibrated by 

 means of the rotating disk, according 

 to the method described by Capp,$ for the five colors: 



* Brace, Astro-Physical Journal, xi, 6. 

 f Murphy, " " " vi, 5. 



jCapp, " " " vii, 25. 



