The following table gives the optical data and chemical analyses of a few typical optical glasses. The list includes common crowns and flints, a typical baryta crown and light flint, and a telescope crown and flint for the better achromatization of objectives, as developed at the Jena works.
The thing most conspicuous here as distinguishing crowns from flints is that the latter have greater relative dispersion in the blue, the former in the red end of the spectrum, as shown by the bracketed ratios. This as we shall see is of serious consequence in making achromatic objectives. In general, too, the values of ν for flints are much lower than for crowns, and the indices of refraction themselves commonly higher.
As we have just seen, glass comes to the optician in blocks or discs, for miscellaneous use the former, three or four inches square and an inch think, more or less; for telescope making the latter. The discs are commonly some ten percent greater in diameter than the finished objective for which they are intended, and in thickness from 1/8 to 1/10 the diameter. They are commonly well annealed and given a preliminary polish on both sides to facilitate close inspection.
Characteristics of Optical Glasses
| Glass | nd | dn (F-C) | ν | Bracketed numbers are proportions of dn | ||
|---|---|---|---|---|---|---|
| D-A´ dn | F-D dn | G´-F dn | ||||
| Boro-silicate crown | 1.5069 | .00813 | 62.3 | .00529 | .00569 | .00457 |
| (.651) | (.701) | (.562) | ||||
| Zinco-silicate (hard) crown | 1.5170 | .00859 | 60.2 | .00555 | .00605 | .00485 |
| (.646) | (.704) | (.565) | ||||
| Dense baryta crown | 1.5899 | .00970 | 60.8 | .00621 | .00683 | .00546 |
| (.640) | (.704) | (.563) | ||||
| Baryta light flint | 1.5718 | .01133 | 50.4 | .00706 | .00803 | .00660 |
| (.623) | (.709) | (.582) | ||||
| Common light flint | 1.5710 | .01327 | 43.0 | .00819 | .00943 | .00791 |
| (.617) | (.710) | (.596) | ||||
| Common dense flint | 1.6116 | .01638 | 37.3 | .00995 | .01170 | .00991 |
| (.607) | (.714) | (.607) | ||||
| Very dense flint | 1.6489 | .01919 | 33.8 | .01152 | .01372 | .01180 |
| (.600) | (.714) | (.615) | ||||
| Densest flint | 1.7541 | .02743 | 27.5 | .01607 | .01974 | .01730 |
| (.585) | (.720) | (.630) | ||||
| *Telescope crown | 1.5285 | .00866 | 61.0 | .00557 | .00610 | .00493 |
| (.643) | (.705) | (.570) | ||||
| *Telescope flint | 1.5286 | .01025 | 51.6 | .00654 | .00723 | .00591 |
| (.638) | (.705) | (.576) | ||||
| * Optical data close approximations only. | ||||||
| Analysis of glasses in percentages | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Si O2 | B2 O3 | Zn O | Pb O | Ba O | K2 O | Na2 O | Ca O | AL2 O3 | As2 O5 | As2 O3 | Fe2 O3 | Mn2 O3 | Sb2 O3 | Mg O |
| 74.8 | 5.9 | ... | ... | ... | 7.11 | 11.3 | ... | .75 | ... | .06 | ... | .06 | ||
| 65.4 | 2.5 | 2.0 | ... | 9.6 | 15.0 | 5.0 | ... | ... | ... | .4 | ... | .1 | ||
| 37.5 | 15.0 | ... | ... | 41.0 | ... | ... | ... | 5.0 | 1.5 | |||||
| 51.7 | ... | 7.0 | 10.0 | 20.0 | 9.5 | 1.5 | ... | ... | .30 | |||||
| 54.3 | 1.5 | ... | 33.0 | ... | 8.0 | 3.0 | ... | ... | .20 | |||||
| 54.8 | ... | ... | 37.0 | ... | 5.8 | .8 | .60 | .4 | ... | ... | .70 | ... | ... | .20 |
| 40.0 | ... | ... | 52.6 | ... | 6.5 | .5 | ... | ... | .30 | ... | ... | .09 | ||
| 29.3 | ... | ... | 67.5 | ... | 3.0 | ... | ... | ... | ... | .20 | ... | .04 | ||
| 55.2 | ... | ... | ... | 22.0 | 5.7 | 7.5 | 5.9 | ... | ... | ... | 3.7 | |||
| 59.9 | 12.7 | ... | ... | ... | 5.1 | 3.5 | ... | ... | ... | ... | 2.7 | 16.1 | ||
The first step toward the telescope is the testing of these discs of glass, first for the presence or absence of striæ and other imperfections; second, for the perfection of the annealing. The maker has usually looked out for all the grosser imperfections before the discs left his works, but a much closer inspection is needed in order to make the best use of the glass.
