WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



275 



.DARK 



Fig. 7. 



LIGHT 



DARK 



precise refractive index determinations obtained, but for most purposes 

 the third decimal place is sufficiently exact. Still greater accuracy can. 

 be had by using plates of different refractive index, the beveled edges 

 of which are so cut as to give the greatest sensibility to slight differ- 

 ences in angular inclination. In this case a lower power objective can 

 be used and, if necessary, one or two units in the 

 fourth decimal place obtained. The phenomena of 

 total reflection take place at the surface of the upper 

 plate as illustrated in figure 5. It is not essential, 

 therefore, that the second glass plate be of the same 

 refractivity as the first. The disturbing interfer- 

 ence fringes, caused by internal reflection, are elimi- 

 nated either by making the slope of the beveled 

 edge of the lower plate different from that of the 

 upper or by grinding it to a matt finish. This method is simple in 

 principle, easy to apply and sufficiently accurate for most purposes. 



Method 6. This method is a variation of method 5. A prism of 

 liquid is formed between a plane-parallel base plate and two small 

 prisms of lead glass cemented to a thin plane-parallel glass plate. The 

 path of the rays is shown in figure 7. The method of measurement is 

 identical with that of method 5 except that here 

 the distance between two boundary lines is taken 

 instead of the distance of one boundary line from 

 the center. The accuracy of this method is not ap- 

 preciably greater than that of method 5; it is, more- 

 over, less convenient to apph' and, to that extent, 

 less satisfactory. 



Method 7. This method is similar in principle to 

 5 but instead of a prism of refractive index higher than that of the liq- 

 uid a prism of lower refractivity is used. A prism of fluorite (n = 1.4:34)> 

 or of silica glass (n = 1.459) is cut so that the inclined edge includes, 

 an angle of 65° with the horizontal face (fig. 8) . The prism is placed 

 on a small glass plate (plate glass) inside a small brass ring cemented 

 with shellac to the glass plate. This ring is then filled to the top 

 with a few drops of the liquid and covered with a thin 

 plane-parallel plate. The path of the rays is indicated ^^.hk ^ light 

 in figure 8. By the use of glass prisms of different re- 

 fractive indices and slopes, it is possible to obtain de- 

 terminations accurate to one or two units in the fourth 

 decimal place. Ordinarily, however, with a 65° prism 

 of fluorite it is possible to measure the refractive in- 

 dices of all liquids ranging in refractive index 1.45 to 1.80 

 with an accuracy of ±0.001. This method requires 

 more liquid than method 5 and is less convenient to use because of the 

 bother of cleaning the liquid from the prism and the brass ring. 



Method 8. This method involves the use of the hollow prism of 

 method 2. Light is admitted as indicated in figure 9 and the inclina- 

 tion of the emergent ray is ascertained as in method 5. This method 

 is comparable in accuracy to 5 but it is not always easy to apply be- 



Fig. 8. 



Fig. 9. 



