Photof/raphic Charting of Ihe Heavens. t!) 



believe, to the ditiicult}' of getting the necessaiy smooth 

 ajid unitbi-m motion oi' the telescope. Still, some of the 

 photographs, viewed in the light of onr present experience, 

 are of high j)romise and encourage fnrtiier expeiiments. 

 Some prints fi-om these pliotographs are on tiie table. These 

 were taken with some of the early gelatine plates made by 

 Edwards in London. A photograph print of the grou]) 

 Kappa Crucifi, will be found interesting to compare witli 

 a print obtained from a ])hotograpli t)f the same object 

 taken with the astrographic telescope. 



The first photograph of a nebnhi taken in the Southei-n 

 Hemisphere, was obtained with the great Mell)onrni' 

 telescope in February 1883. 



Star photography reached the stage of practical success in 

 the iiands of the brothers Henry, of Paris, in 1885. Tlie 

 Paris Observatory liad been for a long time engaged in 

 preparing elaborate charts of the heavens by the ordinary 

 methods of eye observation, but on coining to the regions 

 covered by the milky way, it became evident that by such a 

 method the work would extend over an impracticably long 

 period ; they therefore decided to try tiie photographic 

 metliod, and after numerous experiments, both with respect 

 to optical and mechanical means, as well as photographic 

 processes, they constructed a special instrument with whicl) 

 they succeeded beyond their most sanguine expectations. 

 The photographs depicted a great number of stars not visible 

 in a telescope of tlie same dimensions, and it was soon found 

 that the number of stars imj)ressed on the plate for any 

 particular region, increased almost ad Injinltam with the 

 time of exposure of the plates. Some very important 

 discoveries of celestial objects at once resulted, many 

 interesting physical facts were revealed, and a new and 

 powerful method of astronomical research established, which 

 opens up an immense range of possiliilities. 



The Henrys' instrument was a double telescope equa- 

 torially mounted, one telescope to be used as a guidcr, and 

 the other as the photogiaphic camera, both rigidly connected 

 and moving together. The whole was made to follow the 

 diurnal motion of the earth by clock work mechanism in the 

 usual mannei-, the exceptitm being that this \y<\vt of the 

 in.strument was fitted for nioi-e accurate and uniforai motion 

 than is ordinarily the case. The photogia])hic object glass 

 was 13--i incli(.\s in diametei-, and 13 feet focal length, while 

 the guider telesco})e had an olject glass of less diameter, but 



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