for a double Telescope Objective. 



295 



large numerical values for A and B, but the case of rock salt is 

 worthy of a moment's discussion. The numbers show that it 

 is possible to make a telescope objective of rock salt and ordin- 

 ary telescope flint which, with moderate curvatures, shall give 

 a negative secondary chromatic aberration, that is, which would 

 show the center of a stellar-image purple inside of the focal 

 plane instead of outside as in the familiar case. This peculiarity 

 of rock salt I discovered a long time since. It is of interest 

 since it enables us to eliminate all secondary chromatic aberra- 

 tion by combining a relatively strong binary lens of rock salt 

 and flint with a weak binary of crown and flint, or, in short, to 

 make an absolutely achromatic combination of the three mate- 

 rials named. I had calculated such an objective and com- 

 menced making a pair for spectroscopic work, when the publi- 

 cation of the Jena catalogue suggested combinations of more 

 practical value. 



Table of constants of binary objectives composed of various substances 

 combined with flint, No. 37. 



No. 



A 



B 



- 



¥ 



No. 



A 



B 



— *' 



1 



3-97 



-1-68 



25-5 



24f 



5-91 



— 3-83 



36-4 



2* 



3-85 



— 1-84 



20 







25 



662 



- 4-23 



34-4 



3 



3-86 



— 1-96 



19 



8 



26 



706 



- 4-46 



21-1 



4f 



3-84 



-2-04 



22 



3 



27 



7-58 



- 5-29 



33-2 



5 



4-46 



— 2-04 



25 



3 



28f 



8-64 



— 6-81 



34-5 



6 



4-68 



—221 



25 



4 



29 



10-48 



- 7-98 



21-5 



7 



4-71 



-2 30 



27 



1 



30 



10-6 



— 8-61 



0-5 



8 



4-77 



—235 



23 



6 



53 



5-02 



— 2-94 



23-3 



9 



4-80 



-2-40 



23 







54 



5-65 



— 3-06 



24-1 



10f 



4-91 



— 2-37 



35 



8 



55 



5-45 



— 3-35 



17-2 



11 



499 



— 2-41 



25 



7 



56 



631 



- 3-71 



18-2 



12 



4-67 



-2-43 



26 



2 



57 



5-84 



- 3-72 



20-5 



13 



4-92 



— 2-48 



28 



7 



58 



6-09 



- 3-90 



19-9 



14 



5-00 



-2-58 



17 







59 



6-03 



- 3-92 



18-9 



15 



4 93 



— 2-59 



25 



6 



60 



6-42 



- 4-24 



15-4 



16 



4-63 



— 2-64 



21 



6 



61 



703 



— 4-65 



14-2 



17 



5-21 



— 2-70 



24 



4 



62 



7-53 



— 5-42 



18-8 



18 



5-29 



— 2-76 



22 



5 



63 



10-7 



— 8-10 



11-0 



19 



5-21 



-2-77 



26 



9 



64 



130 



— 10-4 



12-3 



20 



4-70 



-2 95 



22 



2 



65 



12-9 



— 10-2 



5-8 



21f 



5-17 



— 296 



35 



5 



66 



25-8 



— 23.4 



— 13-3 



22f 

 23 



5-42 

 6-21 



— 3-19 



— 3-74 



32 



18 



2 



5 



Rock 



Salt 



11-67 



- 8-52 



- 6-5 



*That in my possession not permanent, f Not permanent. 



Third : that it is easy to select from the table triple combina- 

 tions such that the secondary dispersions shall be completely 

 eliminated. The process would be the following : — Take a neg- 

 ative binary lens of some materials of which the value of —Tc' is 

 large and of such a power, <p', that <p'~k' equals— F for a binary 

 having a small value of this constant. These two binaries com- 

 bined will form a system of power (p—(p r without chromatic 



