+------------------------------------------------------------------------ | | Analysis of glasses in percentages | +----+----+---+----+----+----+----+----+----+----+----+----+----+----+---- | Si |B_2 | | | |K_2 |Na_2| |AL_2|As_2|As_2|Fe_2|Mn_2|Sb_2| | O_2| O_3|ZnO| PbO| BaO| O | O |CaO | O_3| O_5| O_3| O_3| O_3| O_3| MgO | | | | | | | | | | | | | | | +----+----+---+----+----+----+----+----+----+----+----+----+----+----+---- |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. [Illustration: FIG. 40.—Testing Glass for Striæ.] Bad striæ are of course seen easily, as they would be in a window pane, but such gross imperfections are often in reality less damaging than the apparently slighter ones which must be searched for. The simplest test is to focus a good telescope on an artificial star, remove the eyepiece and bring the eye into its place.