MILKY-WAY RECENTLY TAKEN AT SYDNEY OBSERVATORY. 107 



in their own mounting, with the result, that there was no better 

 position than that in which they were originally placed by the 

 maker. I mention this because it may save some one else the 

 trouble of doing it. The next step was to ascertain what exposure 

 was necessary to get in stars down to the 15th magnitude, and I 

 found that three hours was required on a fine night. The telescope 

 mounting works most satisfactorily and there is no trouble what- 

 ever in following the stars perfectly, and the difficulty has 

 been the weather, and that has been bad enough to try one's 

 patience ; the time already spent would have sufficed to take all 

 the photos, had the weather been good, but when the clouds have 

 been absent the moon or a haze has generally taken their place. 

 The moon-light reflected from the atmosphere in S.W. when the 

 moon is in N.E. will fog and spoil a plate in ten minutes, and 

 since the stars require three hours it is useless to attempt taking 

 a photo of them when the moon is shining, and a haze hardly 

 visible is found to double the time of exposure. These are 

 some of the difficulties in the way of taking star photos, and 

 viewed in the light of actual experience they are rather serious, and 

 only to be met by the exercise of a large amount of patience. The 

 work is however intensely interesting, it is throwing an entirely 

 new light on the configuration of the stars in the galactic region, 

 and as will appear presently, some of the features which strike 

 the eye of the observer and have been considered characteristic 

 and definite enough to give rise to the most celebrated speculations 

 of the form and arrangement of the universe in which we move 

 must I fear be classed as optical illusions. 



I have mentioned that the lens I am using only gives a sharp 

 focus over a circle of 5 inches diameter, but by adjustment it may 

 be made passible over a circle of 6 inches, provided one does not 

 examine the star images with too great a magnifying power. 

 Outside of that limit the images gradually pass from circles into 

 crosses, and this distribution of the light over a large surface 

 prevents the smaller stars from recording themselves. I mention 

 this for reasons which will appear presently. Another condition 

 has to be borne in mind, and this is, that the photographic plate is 

 so sensitive, and the lens so quick, that a 1st magnitude star records 

 itself in a small fraction of a second, and a 15th magnitude takes 

 nearly three hours ; the bright stars must therefore be terribly over- 

 exposed and clusters of stars are apt to be run together from the 

 enlargement of the discs of the brighter stars from over-exposure. 

 Still this is not a very serious drawback to my purpose, which is 

 as stated above to picture the great mass of the stars forming the 

 Milky-Way. 



A negative of the Milky- Way is not a very beautiful picture, 

 the plate looks just as if it had been dusted over with black sand,. 



