Archives of ophthalmology and otology . ion acts as if it were a piano-parallel surface,and causes the beam of light entering it to emerge absolutelywithout changing its direction, if it can emerge at all. The anterior and posterior surfaces, ON and PQ (Fig. 2), of theintermediate layer (liquid) are parallel, and therefore also their per-pendiculars, Cj c^ and d^ d^ are parallel, and inclose with the beamof light CD the angle 5; in the anterior prism, the ray of lightforms the angle y, in the posterior the angle y^ with the perpen-diculars to the opposed surface ; then, according to the funda-
Archives of ophthalmology and otology . ion acts as if it were a piano-parallel surface,and causes the beam of light entering it to emerge absolutelywithout changing its direction, if it can emerge at all. The anterior and posterior surfaces, ON and PQ (Fig. 2), of theintermediate layer (liquid) are parallel, and therefore also their per-pendiculars, Cj c^ and d^ d^ are parallel, and inclose with the beamof light CD the angle 5; in the anterior prism, the ray of lightforms the angle y, in the posterior the angle y^ with the perpen-diculars to the opposed surface ; then, according to the funda-mental law of refraction, we have n ^ sm / =— sin ^ Y Itand sin/, = — sin (5 V Consequently BC ^ DE. therefore : / = ;,, sinceboth are < 90°. Coefficients of Refraction, 215 With the perpendicular to the surface MO of the anterior prism, let the beam inclose the angle /5, with that of QR the angle (i^ ; let also : ^ MON — <J PQR — iv ; Then, we have : /3 = ^ — 7^ ; since in the A OBC \ (90^ - /) + (,5 + 90) + ^v = l80^. O p But, for similar reasons R Fig. 2. f5, = / - w ; therefore : /? = /J^; The original angle of incidence be «, the angle of emergencea,; then, sin a = y sin /5 ;sin «, = V sin /5 ;therefore a = mi ; The foregoing conclusions prove that AB d^ EF; Q. E. beam cannot emerge at all, however, if /,is equal to, orgreater than, the critical angle. To find the index of refraction,we now have the following simple equations : i) sin /5 = —sin a V 2) y — ft + zv ;2) n = V sin ;- ; 2l6 ^. HirscJibercr If the constant quantities of the prism apparatus, v and w, areknown, it would only be necessary to determine that angle of incidence «, simultaneously withwhich Y reaches its limit. In order to determine a moreeasily, the combination is connect-ed with a small telescope ; in thefocal plane of the objective there isan opaque diaphragm which has anarrow vertical, transparent slit,bearing a micrometer scale, andcorresponding to the median princi-pa
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