The Astrophysical journal . 3 33 53 06 52 89 52 93 53 05 53 31 53-6o 53-73 152 55-84 55-13 54-o5 53 54,53 26 53 07 53 25 53 38 53 49 53-62 53-64 171 56-05 55-39 54-38 53-9o 53-53 53-56 53-97 54-15 54-27 54-34 down, away from the objective, some 40 mm we get curve C, and at70 mm, curve D. In curve C the minimum focus is nearly at Hy,and in D at \ 4460. Evidently the lowering of the correcting-lenssome 40 mm effects considerable improvement in the color-correctionwithout, as the earlier experiments showed, appreciably enlargingthe image, and the lens has been used in this position almost f
The Astrophysical journal . 3 33 53 06 52 89 52 93 53 05 53 31 53-6o 53-73 152 55-84 55-13 54-o5 53 54,53 26 53 07 53 25 53 38 53 49 53-62 53-64 171 56-05 55-39 54-38 53-9o 53-53 53-56 53-97 54-15 54-27 54-34 down, away from the objective, some 40 mm we get curve C, and at70 mm, curve D. In curve C the minimum focus is nearly at Hy,and in D at \ 4460. Evidently the lowering of the correcting-lenssome 40 mm effects considerable improvement in the color-correctionwithout, as the earlier experiments showed, appreciably enlargingthe image, and the lens has been used in this position almost from thefirst. Although all the data in regard to the complete color-curves aregiven in Tables III and IV, still the actual curves drawn from thesefigures show all the conditions at a glance, and are hence worthgiving. To prevent too great a confusion of lines, the curves forfour zones only (zone plate (B), Fig. 1), of 57, 95, 133, 171 mmradius, are shown here in Fig. 5, the upper curves being of objective 212 /. S. PLASKETT. Fig. 5.—Color-Curves of Four Zones of Objective and of Objective with Corrector alone, the lower of objective and corrector. These curves show at aglance that, in the photographic part of the spectrum, the focus for STAR IMAGE IN SPECTROGRAPHIC WORK 213 the edge of the objective is longer than the focus for the center, thatit has negative spherical aberration. This chromatic difference ofspherical aberration is inherent in two-part objectives of the ordinaryglasses, and the only remedy is to compensate for it by introducingthe correct amount of positive aberration by the , the lower curves show that, instead of compensating forthis chromatic difference, the correcting-lens has, on the contrary,increased it somewhat, and the focus for marginal rays is upward of2 mm longer than the focus for central rays. This agrees almostexactly with the previous determination of the zonal foci of objec-tive and corrector, and is good evidence of th
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
