172 REPORTS ON INVESTIGATIONS AND PROJECTS. 



lished for the same region by a combination of results obtained with the full 

 aperture of 60 inches with those given by diaphragms of 32, 14, 8, and 6 

 inches, a 60° sector-shaped diaphragm, a rotating sector, and a wire-gauze 

 screen of known absorption constant. The agreement of the various magni- 

 tude scales thus derived is so close over a range of five magnitudes that the 

 effect of the change in the diffraction pattern produced by reducing the aper- 

 ture must be regarded as practicably negligible. 



The second method is indirect and involves a determination of the effect 

 on the size of photographic star-images produced by small errors of focus. 

 From a long series of focal and extra-focal plates it was found that, on the 

 average, an error of focus amounting to 0.125 mm. produces an increase in 

 the apparent brightness of the stars photographed of 0.03 magnitude. The 

 tenfold change in the size of the image produced by this error in focus is 

 nearly the same as that produced by the use of a 6-inch diaphragm over the 

 60-inch mirror. While it is probable that the intensity distribution within 

 the diffraction disk corresponding to an aperture of 6 inches is different from 

 that in the cross-section of the full aperture cone at a point 0.125 mm. from 

 the focus, it does not seem likely that this can affect the result greatly. Ac- 

 cordingly it appears that, even with the use of a diaphragm only 6 inches in 

 diameter, the errors introduced will be of the order of 0.03 magnitude. The 

 two methods used for the investigation of the diffraction effect are therefore 

 in agreement in their results. 



The examination of the plates taken for the study of the diffraction effect 

 soon brought to light the fact that the correction to the observed magnitudes 

 depending on the distances of the stars from the center of the plate is very 

 large and variable from plate to plate. This led to an extended investigation 

 of the circumstances which determine the magnitude of this error. The re- 

 sults are still incomplete, though various facts, which may be summarized as 

 follows, have been established clearly : 



(i) The distance correction is practically independent of small errors of 

 focus, and of the kind, strength, time, and temperature of development. 



(2) It depends upon the brightness of the star, or more specifically upon 

 the size of the photographic star-image, being a maximum for the largest 

 images and sensibly zero for the smallest. The effect is to increase the 

 apparent brightness of the stars. 



(3) The magnitude of the correction is also dependent in some way upon 

 the individual star; two stars of the same brightness, and at the same dis- 

 tance from the axis, may require widely different corrections. Thus far it 

 has not been possible to connect this phenomenon either with the color of the 

 stars or with their positions on the plate. 



(4) Aside from the variations in the corrections mentioned under 2 and 3, 

 there are anomalous variations from plate to plate as yet unexplained, though 

 there is some evidence that they may be due to small changes in the figure of 

 the mirror resulting from variations in temperature. 



