l6o CARNEGIE INSTITUTION 



Puiseux have used the great equatorial coude for this purpose with 

 striking success. Their great photographic atlas of the Moon, with 

 the atlas prepared by Weinek from Lick and Paris negatives, pro- 

 vide much material for a study of lunar topography. The recent 

 work of Ritchey with the 40-inch Yerkes telescope, a visual instru- 

 ment adapted for photography by the use of a yellow color screen, 

 has yielded remarkable photographs of the Moon, which should be 

 of service in the study of the smaller details. The smallest mark- 

 ings perceived under good conditions at the telescope are yet to be 

 recorded photographically, but in spite of this fact the best photo- 

 graphs now available are much superior to visual observations for 

 some classes of lunar research. 



Photographs of planets which are at all comparable with xnsual 

 images have not yet been obtained, though the results of Common, 

 the MM. Henry, Schaeberle, W. H. Pickering, and others give rea- 

 son to hope that marked advances may be expected in the future. 

 Judging from the results of recent experience, it is probable that the 

 improvement of both lunar and planetary photographs will come 

 through tiie use of larger focal images. An increase in the sensi- 

 tiveness of photographic plates, unless it involved a corresponding 

 increase in the size of the silver grains, would be of the greatest 

 service in this and in nsost other departments of celestial photog- 

 raphy. For this and other reasons the encouragement of research 

 in photography, which might lead to the improvement of sensitive 

 plates, is greatly to be desired. 



Re.spectfully submitted. 



E. C. Pickering. 



