. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . -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 steel andpivoted at P to swing easily over any portion of the dialplate. In measuri
. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . -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 steel andpivoted at P to swing easily over any portion of the dialplate. In measuring a prism, the position of the indexfinger F will be governed by the difference of the thick-ness of the lens at the points D and D, and the degrees ofthe refracting angle of the prism will be indicated on thescale K by the pointer F. The surgeon is, however, very little concerned with therefracting angles of prisms, except as they are the basis ofthe old system of numbering, which will doubtless soon besuperseded by one in which the number of the prism shall Fig. express in centrads the power which that prism possessesof causing deviation in a ray of light. One of the simplestand most convenient devices for measuring this power isthat suggested by Dr. Maddox. It consists of a strip ofcardboard suspended horizontally on the wall on a levelwith the eyes of the observer. The upper border of thecard (Fig. 44) is marked from right to left with a scale ofdegrees, or rather tangents of degrees, proper to the distanceat which the prism is to be held from the card. In Table 76 SPECTACLES AND EYEGLASSES. VII is given the distance from the right-hand border of thecard of the mark for each degree of deviating angle. Withthe help of this table one may readily construct the scale,using column A if he elect to work at six feet, or column Bif a two-meter range be preferred. To practice this method of prismetry, the glass to betested is held at the proper distance from the card, its apex
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecteyeglasses, bookyear1
