. The clinical use of prisms; and the decentering of lenses . neutralise each other as to have the effect of onlya thick plate of glass. The more they are rotatedfrom this position, the greater is the prismatic effectof the combination, till it reaches its height, whenthe two apices coincide. Cretes, of Paris, mountedtwo such prisms in a circular frame with a handle,so that on moving a button along the handle theprisms revolved in opposite directions at equalrates. This useful instrument is commonly knownas Cretes prism, or the prisme mobile. Eachprism has a refracting angle of 12°, and the in
. The clinical use of prisms; and the decentering of lenses . neutralise each other as to have the effect of onlya thick plate of glass. The more they are rotatedfrom this position, the greater is the prismatic effectof the combination, till it reaches its height, whenthe two apices coincide. Cretes, of Paris, mountedtwo such prisms in a circular frame with a handle,so that on moving a button along the handle theprisms revolved in opposite directions at equalrates. This useful instrument is commonly knownas Cretes prism, or the prisme mobile. Eachprism has a refracting angle of 12°, and the instru-ment is marked accordingly. Landolt has had itmarked in degrees of deviation, and metre we wish to know how strong a prism can beplaced before one eye without creating diplopia,we start at zero, and press up the button till double 48 PRISMS. vision commences, when the strength of the prismcan be read off. By far the neatest form, however, in whichHerschels prisms are put up, is that of the Rotaryprism, designed by Dr. S. R. Risley, of Phila-. Fig. 24.—Risleys Rotary Prism. delphia. It is small and circular, and can be placedin an ordinary trial-frame. On turning the littlebutton the two prisms, each of 15°, rotate in oppo-site directions. The diameter in which the buttonlies coincides with the base-apex line of the virtualprism, however much the actual prisms are virtual prism can be increased from o° to 300(iS°d). The markings indicate the physical angleof the virtual prism, not the deviating angle. Ifthe apex of the front actual prism be to the buttonside of zero, the position of the button indicates theapex of the virtual prism ; if the actual apex be tothe other side of zero, the button indicates the base ROTATING PRISMS. 49 of the virtual prism. Every form of Herschelsprism, however, has some disadvantages. Thediminution in the visual acuity, occasioned by thechromatic aberration and prismatic astigmatism, isin one eye only, and thus the de
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