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.
When the eye is in focus the whole aperture of the objective is uniformly filled with light, and if the disc to be tested be placed in front of it, any inequality in refraction will announce itself by an inequality of illumination. A rough judgment as to the seriousness of the defect may be formed from the area affected and the amount by which it affects the local intensity of illumination. Fig. 40 shows the arrangement for the test, A being the eye, B the objective and C the disc. The artificial star is conveniently made by setting a black bottle in the sun a hundred feet or so away and getting the reflection from its shoulder.
Fig. 41.—The Mirror Test for Striæ.
A somewhat more delicate test, very commonly used, is shown in Fig. 41. Here A is a truly spherical mirror silvered on the front. At B very close to its centre of curvature is placed a lamp with a screen in front of it perforated with a hole 1/32 inch or so in diameter.
The rays reflected from the mirror come back quite exactly upon themselves and when the eye is placed at C, their reflected focus, the whole mirror A is uniformly lighted just as the lens was in Fig. 40, with the incidental advantage that it is much easier and cheaper to obtain a spherical mirror for testing a sizeable disc than an objective of similar size and quality. Now placing the disc D in front of the mirror, the light passing twice through it shows up the slightest stria or other imperfection as a streak or spot in the field. Its place is obvious and can be at once marked on the glass, but its exact position in the substance of the disc is not so obvious.
To determine this, which may indicate that the fault can be ground out in shaping the lens, a modification of the first test serves well, as indeed it does for the general examination of large discs. Instead of using a distant artificial star and a telescope, one uses the lamp and screen, or even a candle flame ten feet or more away and a condensing lens of rather short focus, which may or may not be achromatic, so that the eye will get into its focus conveniently while the lens is held in the hand. Fig. 42 shows the arrangement. Here A is the eye, B the condensing lens, C the disc and D the source of light. The condensing lens may be held on either side of the disc as convenience suggests, and either disc or lens may be moved. The operation is substantially the examination of a large disc piecemeal, instead of all at once by the use of a big objective or mirror.
