ADVISORY COMMITTEE ON ASTRONOMY 159 



or two years. The Draper telescope in Cambridge and the Bache 

 telescope in Arequipa, Peru, are of this type, and have been in con- 

 stant use by the Harvard College Observatory during the last thirteen 

 years. The apertures are eight inches, and each covers a field 10 

 degrees square. The resulting collection of photographs represents 

 each portion of the sky on from one to two hundred nights, and 

 furnishes a historical map of great value. It has not seemed advis- 

 able to go to the expense of measuring and cataloguing a large por- 

 tion of the fainter stars, but when an object of interest is discovered 

 its history is always readily available, as has been shown in many 

 striking cases. Prints on glass of any particular plate in this series 

 are furnished to those who may require them for special studies. 

 There can be no doubt that a systematic examination of these plates 

 would lead to many important results. 



Photography of Nebulce. — The recent development of the reflecting 

 telescope, referred to more specifically in another section of this re- 

 port, has made it possible to secure with comparatively small instru- 

 ments photographs of nebulae greatly surpassing in delicacy of detail 

 the best results previously known. This new era was inaugurated 

 by the late Director Keeler of the Lick Observatory, whose discovery 

 that the typical nebula is spiral in form, and that tens of thousands 

 of these spirals are scattered over the heavens, is of fundamental 

 importance in connection with the great problem of stellar evolution. 

 In spite of the undoubted evidence of motion revealed in the very 

 forms of these nebulae, no actual change in their outlines has as yet 

 been detected. It is very important that an extensive photographic 

 campaign should be undertaken with a very powerful telescope, for 

 the purpose of securing a permanent record of these nebulae as they 

 exist today. For comparison with photographs taken fifty or one 

 hundred years hence such a series of photographs would be invalu- 

 able. 



Photography of Moon and Planets. — Reference has already been 

 made to the photographs of the Moon obtained on wet collodion 

 plates by Rutherfurd. For many years no advance beyond these 

 remarkable results was made, in spite of the advantages arising from 

 the use of gelatine dry plates. It remained for the then recently 

 erected 36-inch refractor of the Lick Observatory to give still sharper 

 photographs in 1890. About the same time, in 1889, similar results 

 were obtained by the Harvard astronomers, enlarging the image in 

 the telescope. The most extensive series of lunar photographs hith- 

 erto obtained is that at the Paris Observatory, where Loewy and 



