. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . ans of Table III, introduced when speaking of the pris-matic equivalent of decentered lenses. The situation ofthe optical center is to be marked upon a spherical lens ofconvenient strength, and the prism to be tested super-imposed. By viewing the corner of a card through thesetwo glasses, as was directed in describing the method offinding the optical center, this center will be found to havebeen carried toward the base of the prism. The positionof this apparent optical center is to be likewise markedupon the spherical
. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . ans of Table III, introduced when speaking of the pris-matic equivalent of decentered lenses. The situation ofthe optical center is to be marked upon a spherical lens ofconvenient strength, and the prism to be tested super-imposed. By viewing the corner of a card through thesetwo glasses, as was directed in describing the method offinding the optical center, this center will be found to havebeen carried toward the base of the prism. The positionof this apparent optical center is to be likewise markedupon the spherical lens, and its distance from the true one7 74 SPECTACLES AND EYEGLASSES. measured. In the left-hand column of Table III find thestrength of the lens used, and on a level with this acrossthe page the distance in millimeters between the true andapparent optical centers. At the head of the column inwhich this measurement is found will stand the strength ofthe prism with which the lens was combined, this strengthbeing expressed in degrees of the refracting angle. For Fig. 43-. A, instance, if having combined an unknown prism with a +7. D. lens we find the apparent displacement of the opticalcenter to be 4 mm., the table shows at a glance that therefracting angle of the prism tested had a value of 30. The refracting angle may be directly measured byadapting the legs of a pair of compasses to the two re-fracting surfaces and then laying the compasses on an INSPECTION AND ADJUSTMENT OF SPECTACLES. 75 ordinary protractor. Various other mechanical contriv-ances have been invented for effecting the same purpose,one of the best of which is represented in Fig. 43. It con-sists of a bed-plate A, upon the front of which is affixed adegree-circle G, and hinged to A at H is the upper plate Bheld up by the spring M, not plainly shown because it isunder B. The upright face-plate C stands at right anglesto B. On top of C is the degree-circle B. The indexlinger F with the lower part D D is made of
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecteyeglasses, bookyear1
