170 REPORTS O^ INVESTIGATIONS AND PROJECTS. 



that the performance of this telescope, both optically and mechanically, would 

 probably meet our highest expectations. Nevertheless, the excellence of the 

 instrument from a photographic standpoint could not be completely deter- 

 mined until provision had been made for protection of the telescope against 

 changes of temperature. This was done by Professor Ritchey, in the sum- 

 mer of 1909, by covering the dome with a canvas screen, supported on a 

 skeleton framework of iron pipe, and also by providing a canopy, made 

 largely of blankets, for inclosing the greater part of the mounting during the 

 day. The effect of the canvas screen was to reduce the average temperature 

 range in the dome from about 21° F. to about 10° F. The canopy reduced 

 the range to only about 3.8° F. The shutter, which is practically air-tight, is 

 closed at the end of the night observations. It is not opened again until 

 shortly before sunset. In the earlier part of the evening a narrow zone at 

 the edge of the mirror, about 3.5 inches wide, is shown by the knife-edge test 

 to be slightly turned up. For work requiring the most perfect definition, 

 this zone is covered by a diaphragm. After midnight the figure of the mirror 

 is usually as perfect as when it was tested in the optical shop. 



As the 60-inch reflector is the largest photographic telescope in use, it is 

 of interest to compare its performance with that of other instruments. Visu- 

 ally, the images of stars, planets, and nebulae obtained with it on a good night 

 are excellent. The star images are very small and sharp and can be focused 

 with great precision. Such an object as the Great Nebula in Orion shows 

 a bewildering variety of detail. Globular star clusters are especially ra- 

 markable, because of the large number of stars made visible by the great 

 light-gathering power. However, the telescope has been so constantly em- 

 ployed for photographic work that it has been necessary to postpone a com- 

 plete visual test until a favorable opportunity presents itself. 



Professor Ritchey 's photographic experiments with the 60-inch reflector 

 have led to a steady decrease in the diameter of the star images on the plate, 

 and an improved plate-carrier, now under construction, will undovibtedly 

 yield still better results. Nevertheless, it may be of interest to give some 

 measures by Professor Scares of the star images on some of these prelimi- 

 nary photographs and on negatives obtained by Professor Parkhurst with 

 the 24-inch reflector of the Yerkes Observatory. The mean diameter of 

 the faintest star images suitable for measurement, on four good negatives 

 (Lumiere Sigma plates) made with the latter instrument, is 3.2". On one of 

 these plates, taken under very exceptional conditions, the diameter of the 

 smallest images is 2.6". For two of these photographs the Yerkes reflector 

 was stopped down to an aperture of 18 inches. 



The focal length of the 24-inch reflector is nearly 8 feet, while that of the 

 60-inch reflector is 25 feet. The mean diameter of the faintest star images, 

 on two negatives (Seed 23 plates) taken with the 60-inch on good nights, is 



