PHOTOGRAPHING THE STARS. 313 



Gill, K.C.B., to whose zeal in the pursuit of his science we 

 are so much indebted for the initiative of the great survey. 

 Dr. Eoberts propounded and proved to astronomers 

 the practicability of making and engraving a chart of the 

 heavens on which many more stars shall be depicted. He 

 has devised a very ingenious and accurate instrument, by 

 which a copy of the stars on the photographic plate can be 

 faithfully engraved on copper. I have had the privilege 

 of seeing and using this apparatus, and hardly know 

 whether to admire most the accuracy of the measurements 

 that can be made by it, or the celerity with which the 

 copper-plate facsimile of the heavens can be obtained. The 

 measures of the distances between stars that can be made 

 with this instrument, either on the photographic plate 

 itself, or on the copper engraved plate, or almost on the 

 impressions taken from that plate on paper, may favourably 

 compare with the most exact and laborious measurements 

 that can be obtained with the heliometer or the micrometer 

 on the actual stars in the heavens. The taking of the photo- 

 graphs being a comparatively simple matter, since an hour 

 with a single telescope will book many thousands of stars, 

 the practicability of the completion of the entire chart of 

 the sky depends on the rapidity with which the plates can 

 be transferred to the copper. Dr. Koberts found that he 

 could easily engrave fifty stars in an hour; so that if 

 twenty engraving instruments were steadily employed for 

 ten years of reasonable working hours a magnificent 

 celestial chart could be completely engraved, consisting of 

 twenty-three millions of stars. This superb undertaking is 

 quite feasible, and every one interested in astronomy will 

 recognise its utility. Here is a splendid opportunity for 



