IA2 CARNEGIE INSTITUTION 



the refractor. In England the evident theoretical advantages of the 

 reflector and the comparative ease of making large specul^e led dur- 

 ing the first iialf of the century to the construction of larger and 

 larger instruments of the reflecting type, which culminated in Lord 

 Rosse's great six-foot reflector, erected at Parsontowu in 1845. The 

 crudeness of the mounting provided for this great mirror, due to 

 the necessity of constructing it without the advantages afforded by 

 modern engineering methods, did not prevent the Irish astronomers 

 from securing many important results ; but even had sensitive pho- 

 tographic plates been available at that time, the advances made in 

 later years could not have been achieved, for a large mirror or ob- 

 jective, however perfect, is of comparatively little value unless pro- 

 vided with a mounting and a driving clock of accurate workman- 

 ship and suitable design. The absence of such mountings undoubt- 

 edly delayed for many years the recognition of the advantages of 

 reflecting telescopes. 



After the completion of Lord Rosse's reflector, attention seems to 

 have been concentrated in large measure on the development of re- 

 fracting telescopes. Notable exception should be made of Sir Wil- 

 liam Huggius, who fully perceived the advantages of the reflector 

 in spectroscopic research and employed such an instrument exclu- 

 sively in his classic investigations ; but through the increased skill 

 of the makers of optical glass and the genius of such men as Alvan 

 Clark, the refracting telescope grew rapidly in size and perfection. 

 The optical improvements were accompanied by corresponding ad- 

 vances in mechanical design, and great perfection has been attained 

 in such modern instruments as the great refractors of the Potsdam, 

 the Lick, and the Yerkes observatories. 



Meanwhile the introduction of photography ifito astronomy and 

 the rapid improvement of photographic processes had revolutionized 

 observatory methods. In a period of rapid development the merits 

 of the reflector, so obvious from an astrophysical standpoint, could 

 not remain long overlooked. In 1883 Draper secured with a re- 

 fractor the first photograph of the Great Nebula in Orion. The 

 first to make extensive use of the reflector for the photography of 

 nebulae was Roberts, whose results form a record of great valtte. 

 The construction of large reflectors was again undertaken. Special 

 reference must be made to the work of Common, who not only con- 

 structed mirrors of three and five feet aperture, but provided them 

 with improved, though hardly adequate, mountings and invented 

 the double-slide plate-carrier. This simple device, which so greatly 



