Fhoioyniphlc ChaiiuKj of flic Heavens. 55 



wlio instantly closes the shutter. During the whole time, 

 ten, twenty, forty or sixty minutes' exposure, the observer 

 has to rigoi-ously watch the star and spicier webs, and check, 

 I)}' means of the accelerator and retarder hand key, any 

 tendency to leave the intersection, and absolutely to keep 

 tiie intersection bisecting the star during the whole exposure. 

 Any failure in this respect results in the photographic images 

 of the stars being * elongated or oval, instead of round, 

 making them difficult for measurement. One plate being 

 thus exposed it is removed, and the instrument set on the 

 next guide star, and so on. All changing of plates has of 

 course to be done in the dark room, and the observing room 

 itself must be kejit moderately dark during the exposure. 

 The development of the plates with us is usually done on 

 the following day. 



So tar as the Melbourne Observatory is concerned, none 

 of the I'egular charting has been commenced yet, although 

 for four months past we have been busily engaged in 

 necessary ]»relirainary and ex])erimental work. It was not 

 until the end of March that the (Jentral Congress detinitel}^ 

 decided many of the princi{)al questions, and the final 

 instructions have not reached us even yet, nor indeed have 

 the Reseaux, &c., and the appliances for their use. Never- 

 theless, there are so many preliminary difficulties to be 

 surmounted, and so much to learn regarding the effects of 

 varying atmospheiic conditions on the photographic process, 

 and especially as regards time of exposure, that there has 

 been no time actuall}^ lost yet. The work will occup\' 

 several years — tive at least, and probably more. Concerning 

 the more purely photographic part of the woi-k, the relation 

 of magnitudes of the stars to the size of their photographic 

 images on the plates, and the effect of fluctuating conditions 

 of tiie atmos])here, I hope to be al)le on some future occasion 

 to conti-ibute some interostino: facts. 



