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WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



cause of the wide angled prisms which are required (between 70 and 

 80°). It is, however, less convenient than method 5, especially as the 

 liquid in the flat lying prism tends to spread and to smear the glass 

 plates and prism as well. For a range of refractive indices 1.50 to 

 1.75 at least two such prisms are necessary unless the prism be momited 

 on another prism so that its lower surface is inclined at an angle with the 

 horizontal. Thus a 60° prism of liquid is obtained by grinding the 

 one face of the prism at an inclination of +22°, and the second face at 

 — 38° with the horizontal base. The inclination of the emergent ray 

 for a liquid of refractive index 1.50 is then +10° 06', while for a 

 liquid of refractive index 1.75 it is —10° 03'. Under these condi- 

 tions the accuracy is identical with that of method 5, namely ±0.001. 

 Although this method is less simple than method 5, it is theoretically 

 superior to it in one respect; namely, it is free from any error intro- 

 duced by the dispersion ot the glass prism; practically this is of no 

 significance because the refractive indices of the glass prism are definitely 

 knowni for the different wave lengths. 



Method 9. The device on which this method is based is illus- 

 trated in figure 10. It consists of a thick glass plate with one "edge 

 beveled by two flat surfaces, the one inclined 25°, the second at 60° to 

 the horizontal base (fig. 11). A plane parallel glass plate (8x15x1 mm.) 



Fig. 10. 



Fig. 11. 



is clamped to the 60° surface by a screw clamp; while on the 25° sur- 

 face a plane-parallel plate (6x8x1 mm. and silvered on the back) rests 

 with its edge in contact with the first plate (fig. 10). A drop of 

 liquid is held by capillarity in place as a liquid prism between the 

 two plates. The rays from a thin total-reflecting cover glass prism® 

 in the rear focal plane of a 16 mm. objective are reflected, after re- 

 fraction in the liquid, back into the objective where the illuminated 

 line image of the cover glass prism edge is viewed simultaneoush' 

 vnth the prism edge, through a Bertrand lens and positive eye piece 

 fitted with the micrometer scale as described in method 5. The 

 exact position of the bright line is read off directly on the microm- 

 eter scale and thus the refractive index of the liquid ascertained 

 for the particular wave length of light employed. As in the fore- 



« J. Wash. Acad. Sci., 3: 234. 191.3. 



