Characteristics of striæ in optical glass . get a record of striae, it is necessary to project themupon a photographic plate, a result which can be reached in anumber of ways. PousHED SivABS OR Prisms need simply to be placed betweena white screen and a strong, approximately a point, source oflight in order to have a shadow of the striae appear on the screen. 1 In testing with an immersion tank it is very convenient to use a second pinhole in place of the opaquedisk, for in that case the position of the image of the pinhole is fixed. With a slab having polished faces,any angle between these fa
Characteristics of striæ in optical glass . get a record of striae, it is necessary to project themupon a photographic plate, a result which can be reached in anumber of ways. PousHED SivABS OR Prisms need simply to be placed betweena white screen and a strong, approximately a point, source oflight in order to have a shadow of the striae appear on the screen. 1 In testing with an immersion tank it is very convenient to use a second pinhole in place of the opaquedisk, for in that case the position of the image of the pinhole is fixed. With a slab having polished faces,any angle between these faces refracts the images with consequent inconvenience to the observer. Smith, Bennett,Merritt StricB in Optical Glass 79 The photographs of the six prisms shown in Fig. 3 were taken inthis way. The results of the projection can be rendered somewhat morestriking by using a projection lens, as in Fig. 4. The two photo-graphs of Figs. 5 and 6 show the results so obtained. If theglass be so set that its image is sharply in focus on the photo-. 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
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Keywords: ., bookcentury1900, bookdecade1920, bookidcharacterist, bookyear1920
