Characteristics of striæ in optical glass . FiG. 5.—Strict in slab too close to projection lens for best focus graphic plate, the striae are very faint; but if the image be throwna little out of focus, the striae show up clearly. The slab of glasswas moved a little too close to the projection lens for good focusin Fig. 5 and was a little too far away for good focus in Fig. 6,the slab being moved about 7 cm. between the two stria which appears dark with the projection screen on one. Fig. 6.—Strice in slab of Fig. 5 too far from projection lens for best focus side of the focus will a
Characteristics of striæ in optical glass . FiG. 5.—Strict in slab too close to projection lens for best focus graphic plate, the striae are very faint; but if the image be throwna little out of focus, the striae show up clearly. The slab of glasswas moved a little too close to the projection lens for good focusin Fig. 5 and was a little too far away for good focus in Fig. 6,the slab being moved about 7 cm. between the two stria which appears dark with the projection screen on one. Fig. 6.—Strice in slab of Fig. 5 too far from projection lens for best focus side of the focus will appear bright when this screen is moved tothe other side of the focus. This change in the appearance of theimage here shown is characteristic. By this means it is alwayspossible to distinguish between a scratch or a bubble and a striae in the slab photographed in Figs. 5 and 6 are prominentand are readily seen with the unaided eye. 8o Scientific Papers of the Bureau of Standards [VoUx6 i^NS a/voF/? rr^T P/?OJ^CrJON LeA/>5 A set-up of essentially the same type as that shown in Fig. 4was devised by Dr. S. M. Burka of this Bureau in collaborationwith Dr. M. Zwillinger of the optical shop annex of the Naval GunFactory at Rochester, and has been used for routine testing of binocular prisms, one girltesting several hundred in aday with very little Stri^ in a Lens canbe detected by simply catch-ing their shadow on a diffus-ing screen placed betweenthe lens and the
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