Characteristics of striæ in optical glass . nditionsare simpler. With the latter instrument the position of the edgeof the illuminated field indicates the refractive index of the lowersurface of the test slab where it is in contact with the monobromo-naphthaline lying between it and the high-index prism of therefractometer (Fig. 18). Scientific Papers of the Bureau of Standards \ By using the knife-edge attachment with which the instrumentis provided the field may be reduced, in the case of a monochro-matic source of light, to a single, narrow band. This is becausethe knife edge cuts off
Characteristics of striæ in optical glass . nditionsare simpler. With the latter instrument the position of the edgeof the illuminated field indicates the refractive index of the lowersurface of the test slab where it is in contact with the monobromo-naphthaline lying between it and the high-index prism of therefractometer (Fig. 18). Scientific Papers of the Bureau of Standards \ By using the knife-edge attachment with which the instrumentis provided the field may be reduced, in the case of a monochro-matic source of light, to a single, narrow band. This is becausethe knife edge cuts off oblique rays, allowing only those whichare practically parallel to its lower face to enter the test slab. Byadjusting the knife edge this band may be made to appear as a spec-tral line. If now, as seen in Fig. 19, there is a stria in the lowerface of the test slab, a second spectral line appears on the one sideor the other of the main line, depending on whether the stria hasa higher or a lower refractive index than the rest of the glass.^. Fig. 19.—Stn(B in Pulfrich test slabThe index of the stria in this sample differs from that of the surrounding glass by In measuring the index of the stria by this method it is necessaryto distinguish between the doubHng of the line by the stria and thefrequently observed doubHng by accidental causes. Certain testsenable the following distinctions to be drawn: 1. If the doubHng be due to a stria, any adjustment of theinstrument which eHminates the second Hne causes the first alsoto vanish, but not if the second line arises from an accidentalcause. 2. A small shift of the test slab on the table will often eHminateHues, while a relatively large shift is usually necessary to carry astria out of the field. 3. Finally, if the doubHng arises from a stria, the effect isobserved to appear for certain definite positions of the slab as itis shifted back and forth on its table, which is not the case withaccidental doubHng. By a combination of
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