MOUNT WILSON SOLAR OBSERVATORY. 165 



Shortly before his departure M. Chretien spent some weeks in Pasadena 

 and carried on investigations on the law of distribution of stars on some of 

 Mr. Ritchey's photographs of globular star clusters. He also did consider- 

 able theoretical work on the problem of methods of increasing the field of 

 the reflecting telescope. 



THE SIXTY-INCH REFLECTOR. 



The performance of the 60-inch reflector during the past year has been 

 extremely satisfactory from the standpoint both of photographic and of 

 visual observations. Although comparatively few nights have been devoted 

 to visual work, the instrument has been employed most successfully by Mr. 

 Hale and others of the staff in observations of the planets and certain star 

 clusters and nebulae. Visual observations were also made by many of the 

 visiting astronomers present at the recent meeting on Mount Wilson of the 

 International Solar Union. 



A new double-slide plate-carrier of improved design was completed in the 

 autumn of 1909. With this instrument two guiding eye-pieces are used on 

 opposite sides of the center. In this way any slight rotation of the field can 

 be detected immediately and corrected by rotating the bronze plate which 

 carries the guiding eye-pieces and the plate-holder. Two fine screws with 

 graduated heads are provided for this purpose. The eye-pieces give a mag- 

 nification of about 750 diameters. 



The plates used are 3.5 inches (89 mm.) square, and are sufficiently large 

 to cover all of the field which is free from serious distortion. The plate- 

 holder is so designed that it can be removed quickly and replaced as fre- 

 quently as desired during long exposures, thus allowing of frequent refocus- 

 ing by means of the knife-edge method. The position of the plate-holder is 

 defined by small hardened-steel surfaces, so that it returns accurately to its 

 original position. Since, as was stated in the last report, changes of focal 

 length have been found to be due almost entirely to the expansion and con- 

 traction of the metal tube of the telescope, it is evident that no change of 

 scale is introduced, and hence no injury to the definition of the star-images, 

 by frequent refocusing. With this instrument Mr. Ritchey has obtained on 

 his best negatives, taken with Seed "23" plates, perfectly round star-images 

 i // .03 in diameter after an exposure of 11 hours. 



A number of photographs of the stars near the north pole have been 

 secured by Mr. Fath during the year and sent to Professor Pickering for 

 use in his determination of a scale of photographic stellar magnitudes. One 

 of these was obtained with an exposure of 4 hours. Although the surface 

 of the 60-inch mirror was not in the best of condition at the time this photo- 

 graph was taken, in the opinion of Professor Pickering stars of about the 

 twentieth magnitude appear upon the negative. 



