CURVED PHOTOGRAPHIC PLATES. 



Bv F. A. BELLA^fY. Hon. M.A. COxon.). F.R.A.S. 



A SHORT note on this subject may be of interest. 

 From the earhest days in photographic astronomy 

 one of the difficulties to be met and overcome was 



Two exposures were made on the same plate, the left hand one with the 

 plate curved, the other with it flat. 



the curvature of the field, or the rapid falling off in 

 the definition or good focus in the images of the 

 stars away from the centre of the photographic plate. 

 It is obviously of the greatest importance, 

 for accuracy and utility, for uniformh- 

 good quality definition to be obtained over 

 as large an area on the plate as possible. 



To an astronomer the reduction of the 

 aperture of the photographic lens is not 

 permissible, or, at least, usually yer\' 

 inadvisable for stellar \vork, as the 

 maximum aperture is required in order 

 to reduce the time of exposure as much 

 as possible. 



Though curved plates have i 

 been suggested and used with 

 overcome the want of good focus be}ond 

 a very limited area, they have never 

 found favour with astronomers, for various 

 reasons, among these being the difficult\' 

 of making them uniformly suit the area 

 of good focus simultaneously at the 

 centre and edges, the great cost of 

 making satisfactory plates (uniformh- 

 coating them), greater risk of breakage, 

 difficulty of measurement, storage, and so on. 

 When the first Conference in connection with 

 the Astrographic Survey was held in Paris, in 18S7, 



one of the subjects proposed and discussed was 

 the use of curved plates for the Survey : the plan 

 was rejected as not feasible. In twent\-three years 

 knowledge has advanced, and the utilization 

 of methods and appliances — common in 

 other branches of science — has greatly 

 increased ; so that what was impossible or 

 inconvenient years ago is now rendered 

 more convenient and available for use. 



The suggestion to use curved plates 

 has been brought up again. This time, 

 from a personal acquaintance with the 

 \\orker and his excellent work, we may 

 say that there is a much greater chance 

 of the method being tried under every 

 condition that promises success, which, 

 we hope, will ultimately be achieved b\- 

 Dr. J. H. Metcalf. 



The lens he is using is a Petzval 



Doublet, and the focal length is seven 



times the aperture and gives a scale of 



picture of 90" to 1""". WJth this lens he 



has found by re-focussing for various 



parts of the plate that all parts of a 



10-in.x8-in. plate, equal to an area of 5 



degrees square, can be brought into good 



focus and excellent star images obtained. As the 



lens can do so much by merelv altering the focus 



gradually bet\\een the centre and edges, the extreme 



n the past 

 a view to 



Figure 2. The Edge of the Plate. 

 Here there were two exposures as in Figure 1. 



amount to be allowed for in changing the focus for 

 these two positions is only three one-hundredths of 

 an inch (0"8""") ; it seems quite possible to bend a 



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