'INDEX OF R.KFRACTION KO that the prism is immersed in a bath of the liquid und< examination and the index is road on a scale within. The compensator for the dipping refraetometor / ^& consists of a single Amici prism (C of Fig. 35) rotated > Jp by the milled ring Y. The range of compensation is thus less than that of the double compensator but it is sufficient for this refraetometer, whose1 range for indices of refraction is comparatively narrow. In Fig. 35 two points are arbitrarily selected to represent the entire surface of the prism when* light enters. f/f/' is the ray of grazing incidence refracted and focused at (V, which boars the scale, a and a' represent rays onto ring at any other incident angle*, focused at A. Above is a projection of the field. At the right of (V the* field is (lark because the critical ray, refracted as it enters the prism, can make no greater angle of refraction for the given prism-fluid combina- tion. When the fluid is changed for another having a different index of refraction that angle of refrac- tion for the critical ray is changed and the border line within the field is shifted. Putfrich Refractometer.—Thin instrument is espe- cially adapted to use, with volatile liquids, although it is suitable also for exact determinations of index of refraction of any liquid whatever. The optical principle IB exactly the name an that of the Abbf. instrument, light entering the fluid-prism system *tt grazing incidence and the critical angle of total refl< c- tion being measured. The liquid in placed in a glass cup which is cemented to the top of the* refracting v/ / prism, and which may be covered to prevent evapo- /II ration of the liquid. Monochromatic light is used $^ and an observation telescope in swung upon an arm ^ :ifl ....„ in such a way as to bring the divinicm between light Dippin« r**- and dark fields upon ite crosn hairs. A circular nealc* J^7nTf