MOUNT WILSON OBSERVATORY. 



245 



The path of the light in these experiments is not far above the ground, and 

 it is possible that in the winter, with the ground covered with snow, conditions 

 would be more favorable. Accordingly, the apparatus will be kept in readi- 

 ness for observations at that season. It is also possible that a site might be 

 selected in which the average path would be much farther from the ground- 

 level. In the event that these two possibilities fail, recourse may be had to 

 Michelson's original plan of a pipe-line exhausted of air to about 0.01 atmos- 

 phere. 



A SCALE OF ASTRONOMICAL SEEING. 



The importance of an objective scale for conditions of seeing has become 

 especially great in connection with such refined observations as those of 

 stellar diameters by interference methods. At the suggestion of Professor 

 Michelson, an iris diaphragm was placed not far from the focus of the 60-inch 

 reflector when used in the Cassegrain form. A series of observations was 

 made by Mr. van Maanen under different conditions of seeing, the aperture of 

 the diaphragm being varied, thus varying the effective aperture of the tele- 

 scope, until diffraction rings appeared. 



A comparison of the results obtained in this way, with the usual visual 

 estimates of seeing based on a scale of 10, shows that for practical purposes the 

 simple relationship may be used, 



^' = 1+0.15 d 



where *S' is the seeing on a scale of 10 and d is the effective diameter of the 

 mirror. Perhaps a more rational formula, and one which has the merit of 

 passing the zero and giving a finite value for an infinite diameter d, is 



S" = 10 (l-e-o-os-^) 



The following table shows the calculated values S' and S" based on these 

 two formulae, compared with the visual estimates of seeing: 



THE MICHELSON AND MORLEY EXPERIMENT. 



During the months of November and December, Professor Dayton C. Miller, 

 of the Case School of Applied Science, repeated on Mount Wilson his observa- 

 tions of ether drift, using the same interferometer with a light-path of 32.24 

 meters, which he had employed for his previous investigations in March and 

 April. In the hope of eliminating magnetic effects and reducing those due to 

 radiant heat, the steel frame of the interferometer was replaced by concrete, 

 and aluminum and brass supports were constructed for the optical parts. 

 Observations were made at various times during both day and night and 42 

 complete sets were secured. 



The results showed that although the displacements of the fringes pre- 

 viously observed for each complete rotation of the interferometer were still 



