SOME RECENT ASTRONOMICAL EVENTS. 155 



ica to complete the spectrograph ic survey of the heavens, and the great 

 equatorial at Yerkes Observatory has within a few months been fitted 

 with a new stellar spectrograph of the greatest perfection. 



2. RECENT ADVANCES IN ASTRONOMICAL PHOTOGRAPHY. 



It was formerly the custom, in the time of Sir William and later of 

 -Sir John Herschel, to employ reflecting telescopes for stellar observa- 

 tions. With the more recent high development of refracting tele- 

 scopes, mirrors became superseded largely by lenses for the most 

 refined work. It is well known to what extraordinary size and perfec- 

 tion telescope objectives have risen, so that in the United States alone 

 we have perhaps as many as half a dozen of over 2 feet clear aperture, 

 the largest being the 40-inch equatorial of the Yerkes Observatory. 

 But while the substitution of refracting for reflecting instruments thus 

 went on, the introduction of photography in astronomical work gave 

 an impetus which has since led to the revival of the use of reflectors. 



The advantage of the latter is due in part to the fact that reflecting 

 instruments bring all raj^s of whatever wave length to the same focus, 

 while refractors can only be corrected to bring a certain limited num- 

 ber of wave lengths to a focus at an} T given plane. When refracting 

 instruments are constructed for visual purposes it is customary to cor- 

 rect the lens in such a way that the rays which affect the eye most 

 intensely shall be brought to a sharp focus, neglecting so far as is 

 necessary the violet rays which are most active photographically. It 

 will be readily seen therefore that a visual refracting telescope is not 

 suitable for the most exact photographic operations. Hence it has 

 been the custom, followed at the Lick Observatory, at the Astrophysi- 

 cal Observatory at Potsdam, and at many other observatories where 

 great refractors are employed, to have an additional lens, either used as 

 a corrector for the visual objective, or wholly substituted for it, to be 

 employed solely for photographic purposes. This has necessitated a 

 very great initial expenditure of money as well as no inconsiderable 

 waste of time and danger to the instruments in the substitution of 

 lenses, as the instrument is changed from visual to photographic uses. 



The fact that a reflecting telescope with all its appurtenances, of equal 

 light-gathering power to a great refractor and without the defect 

 <>f chromatic aberration, can be made at a small fraction of the cost 

 merely of the lens itself, has therefore led several large observatories 

 to yield their great equatorials chiefly to visual and spectroscopic 

 purposes, supplementing their equipment for stellar photography by 

 the use of a reflector with whoil} 7 separate driving mechanism and 

 dome. 



Against the very great advantage of a reflector in point of cost. 

 however, there is to be offset the fact that the extent of the field 

 where the definition remains good at the focus of a large lens is far 



