Characteristics of striæ in optical glass . i- f- 2-4- 6-/6-32 64- ^fCJ. Fig. 3.—Striated prisms and their corresponding star images 78 Scientific Papers of the Bureau of Standards [Vol. i6 tiirers of glass was confining himself to it. For the more generaluses striae which are heavy enough to be injurious can undoubtedlybe detected in this way. A little refinement in the operation consists in placing anauxiliary lens of perhaps 8 to lo inches focal length between theeye and the glass. If a fairly small sotirce—an incandescentlight lo or 15 feet off answers very well—be used at some distancefro
Characteristics of striæ in optical glass . i- f- 2-4- 6-/6-32 64- ^fCJ. Fig. 3.—Striated prisms and their corresponding star images 78 Scientific Papers of the Bureau of Standards [Vol. i6 tiirers of glass was confining himself to it. For the more generaluses striae which are heavy enough to be injurious can undoubtedlybe detected in this way. A little refinement in the operation consists in placing anauxiliary lens of perhaps 8 to lo inches focal length between theeye and the glass. If a fairly small sotirce—an incandescentlight lo or 15 feet off answers very well—be used at some distancefrom the observer, and if the light pass through the slab and thelens in succession and the eye pupil be placed at the image of thesource, the striae in the glass will be easily visible. Of coiursestriae in the lens will show up likewise, but a rotation of the lensenables the observer to distinguish between lens and plate. Ifthe refinement of this apparatus be carried one step farther and. Fig. 4.—Projection of the striae in aflat slab of optical glassThe plate is placed a short distance out of focus for the image of the slab an artificial star with a collimator be substituted for the incandes-cent light, an apparatus is obtained which will probably detectall striae which could ever have an injiurious effect in any opticalinstrument. It is possible to carry the refinement another step farther,using a knife-edge or small opaque disk to hide the image of thestar, in which case only the light that is deviated by the striaereaches the eye, whence greater contrast is obtained.^ 2. PROJECTION OF STRLS In order to 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 co
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