Hastings — Aberration for a double Telescope Objective. 291 



Art. XXXI. — A General Method for determining the Secon- 

 dary Chromatic Aberration for a double Telescope Objec- 

 tive, with a description of a Telescope sensibly free from 

 this defect ; by Charles S. Hastings. 



A former paper by the writer* described a method of find- 

 ing the practicable combinations of three kinds of glass to pro- 

 duce an objective without secondary chromatic aberration. 

 The result of the investigation showed that there had been 

 several optical glasses studied and described, notably one by 

 Fraunhofer and one by Van der Willigen, which would meet 

 the practical requirements of the problem in combination with 

 types of optical glass now readily procurable. Since publish- 

 ing that paper the writer has made very many experiments with 

 a large variety of glasses, not only those made specifically for 

 optical purposes but many others, without, however, finding 

 any really useful combinations until recently. But within a 

 few years the variety of material at the command of the work- 

 ing optician has been enormously increased by the invaluable 

 labors of Dr. Schott and Professor Abbe ; and within the last 

 two years the results of their investigations have been put at 

 the command of the working optician by the manufacturing 

 firm of Schott and Company of Jena. What Professor Abbe 

 and Dr. Zeiss have accomplished with these newly acquired 

 means in the improvement of the microscope is a most 

 interesting history familiar to all, but their utility in the 

 way of improving the telescope has not been thoroughly in- 

 vestigated. With a view to this investigation the writer pro- 

 cured a number of these glasses of the most notable optical 

 peculiarities just after the catalogue of Schott and Co. came 

 into his possession, omitting, however, those which were known 

 not to be permanent under ordinary atmospheric exposure ; for, 

 although small lenses of microscope objectives may be used 

 with such care that the deterioration of the surfaces need not 

 be serious for many months or even years, the larger and far 

 more expensive lenses of the telescope must be permanent or 

 their final cost would render them wholly impracticable. This 

 purely practical limitation was imposed notwithstanding that 

 the manufacturers recommended a number of combinations 

 which would yield greatly diminished secondary dispersion if 

 it were dispensed with. 



The method employed in the paper cited above is rigidly 

 accurate, but very laborious in its application. It required the 

 making of a very accurate prism of each variety of glass and 



* This Journal, III, vol. xviii, p. 429. 



Am. Joub. Sci— Third Series, Vol. XXXVII, No. 220.— April, 1889. 

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