pe AV STARfi V REALMS. 



the test employed will show the peculiarities of a photo- 

 graphic objective. The instrument was directed on an 

 artificial star a couple of hundred feet distant. The star 

 was merely a reflecting bead, illuminated by a spectrum 

 obtained by passing a beam from a small incandescent 

 electric light through a prism ; any part of the spectrum 

 could be cast upon the bead, and thus stars of varied hues 

 could be observed in the telescope. Were the objective 

 designed for visual purposes, the focus of a star near the 

 extreme red should coincide with the focus of a blue star, 

 while the foci of all the other stars would be in the im- 

 mediate vicinity. For the photographic telescope, how- 

 ever, the essential point is that all the bluish stars shall 

 be brought practically to the same focus, and this being 

 so for the visible stars, the invisible foci of photographic 

 light will be all sufficiently concentrated. It should, how- 

 ever, be understood that the only final test of an objective 

 consists in the nature of the photographs it produces. 



Supposing that the photographic telescope, either 

 reflector or refractor, has been prepared, the practical 

 conduct of the work demands a few words of explanation. 

 It is of course essential that the telescope be presented to 

 the same part of the sky throughout the entire duration 

 of the exposure. This condition is complied with by a 

 simultaneous observation of the heavens through a visual 

 telescope rigidly attached to the photographic tube with 

 the axes of the two instruments parallel. The clock 

 motion of the equatorial must be of the highest order of 

 excellence, but notwithstanding the exquisite refinement 

 obtained by the electrical control of the driving clock, 

 it is impossible to dispense with simultaneous watching 



