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VIII. 



THE OONSTEUGTION OF TELESCOPIC OBJECT-GLASSES FOE THE INTEE- 

 NATIONAL PHOTOGEAPHIC SUEYEY OF THE HEAVENS. By SIE 

 HOWAED GEUBB, M.A.I., F.E.S., Hon. Sec. Eoyal DubHn Society. 



[Eead Notembee 19, 1890. 



In the construction of telescopic objectives for visual use, it is necessary to satisfy 

 two conditions only, assuming, of course, that the quality of the material and 

 the workmanship of the surfaces be perfect. These two conditions are, that the 

 chromatic and spherical aberrations be corrected as nearly as possible. In the con- 

 struction of objectives suitable for photography, it is necessary to satisfy a third 

 condition, viz. freedom from coma in the lateral pencils, in order to obtain as 

 perfect a field as possible. 



If the lateral images of an ordinary telescopic objective be examined, it will be 

 found that at a very small distance from the centre of the 

 field of view, say 20' or 30', a very sensible coma is appa- 

 rent : this coma is toward the axis, and gives the images 

 the appearance shown in fig. 1. Owing to the small field 

 usually employed, or necessary to be employed, in astro- 

 nomical telescopes, this coma is rarely seen, but observers 

 are quite familiar with the appearance of a star, when the 

 objective is out of adjustment, and requires alteration 

 to place its axis coincident with the axis of the tube. The appearance in that 

 case is the same as that mentioned above, for the images that the observer then 

 sees in the centre are really due to lateral pencils ; and it is by the position of this 

 coma that the observer determines the direction in which the necessary adjust- 

 ment must be made, in order to utilize the central pencil, and get the best results. 

 In determining the best form to be given to these objectives, it was necessary to 

 keep in view the ultimate purpose for which the photographs obtained by them 

 were intended. Some forms of objectives give a very widely-spread coma, 

 with a very distinct nucleus in the lateral images, and photographs taken with 

 such objectives are sometimes very deceptive. For, if no large stars be upon 



TEAKS. EOT. DUB. SOC, N.S. TOE. IT., PAET Till. 3 TJ 



Fig. 1. 



