The Astrophysical journal . nd thus confine the actionto the visual part of the spectrum. A deep yellow screen with planeparallel surfaces was used in contact with the plate. Although thepencils from the zone plate are displaced slightly on passing throughthis screen, these displacements are proportional, and the onlyeffect will be to lengthen the focus for all the zones by the same amount,about one-third the thickness of the screen, without in the least alter-ing the relative positions of the pencils. An exposure of about aminute on Capella, through the screen, with the plate from 60 to icx?m
The Astrophysical journal . nd thus confine the actionto the visual part of the spectrum. A deep yellow screen with planeparallel surfaces was used in contact with the plate. Although thepencils from the zone plate are displaced slightly on passing throughthis screen, these displacements are proportional, and the onlyeffect will be to lengthen the focus for all the zones by the same amount,about one-third the thickness of the screen, without in the least alter-ing the relative positions of the pencils. An exposure of about aminute on Capella, through the screen, with the plate from 60 to icx?mm from the focus, gives a negative of good intensity in which theimages of the pencils are quite round and free from any noticeablespectral elongation, thus allowing accurate measurement. Five sets of extra-focal exposures were made in the visual part ofthe spectrum, and the mean values resulting from the measurementand reduction of these plates are given in Table II and plotted graphi- STAR IMAGE IN SPECTROGRAPHIC WORK 207. Fig. 3.—Zonal Differences of Focus callvjn curve F of Fig. 3. An examination of this curve shows thatno point or focus is at a greater distance than o. 2 mm from the position 2o8 /• 5. PLASKETT of mean focus, shown by the horizontal line, except a small regionnear the center of the objective, which has a longer focus. The effectof this region on the performance of the objective must, however, beexceedingly small, owing to its small area, less than one-tenth of theobjective, and to the weak convergency of the pencils proceedingfrom it. In fact if Hartmanns criterion T1 as to the quality of anobjective be computed from the above mean values, it is found to According to this classification an objective is moderately(massig) good when T is greater than , good when T is and , and exceedingly (hervorragend) good when T is lessthan In the ideal, absolutely zoneless objective T is o. Evidently the objective when used visually is o
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
