WRIGHT: MEASUREMENT OF REFRACTIVE INDEX 



273 



reflection whereby the field is divided into a brightly illuminated half 

 and one slightly less bright. In this case the boundary line is often 

 very difficult to see and accurate settings can rarely be made. If a 

 cover glass be used to cover the drop of liquid, interference bands appear 

 in the field and increase still further the difficulty of making satisfac- 

 tory settings. These interference bands can be eliminated by tilting 

 the cover glass slip or by grinding its surfaces to a matt velvet finish. 



There is, however, a still better method, by means of which the 

 advantages of the method of grazing incidence can be had even on single 

 drop of liquid. To accomplish this a sheet of ordinary thin tin foil, 

 used for WTapping purposes, is first prepared by pressing it with a 

 rubber eraser against a finely ground glass plate. By this process the 

 surface of the tin foil is covered with fine pits, too fine to be distinguished 

 by the unaided eye but relatively' large when compared with the wave 

 length of light. If now a drop of liquid be placed on the plane surface 

 of the glass hemisphere of the refractometer and then covered by a 

 sheet (10 x 20 mm.) of the prepared tin foil and monochromatic light 

 be reflected into the hemisphere at an angle less than the critical angle 

 (fig. 3), then the irregular surface of the tin foils reflects and diffracts 

 the light rays back into the glass hemisphere at all possible angles 

 with the result that the effect is strictly that of grazing incidence 

 (light and dark field) and without disturbing interference bands. This 

 method has proved of the greatest assistance in this laboratorj^ in the 

 routine measurement of the refractive indices of liquids. 



REFRACTIVE 

 LIQUID 



■TIN FOIL 



Fig. :i. 



Fig 4. 



New petrographic enteroscope stage refractometer s. In many instances 

 petrographers are unable to use the immersion methods for the measure- 

 ment of the refractive indices of mineral grains, because of the lack of 

 a suitable refractometer and, as a result, neglect such measurements 

 altogether, even though they recognize the importance of quantitative 

 optical work in petrography. In view of this situation, it has seemed 

 to me worth while to try to devise a simple method for use with the 

 petrographic microscope b}' means of which the refractive index of 

 any liquid can be measured with an accuracy of ±'0.001, which is 

 adequate for ordinarj^ determinative purposes. A number of such 

 methods are suggested below and with one or two exceptions, are based 

 on the fact that the petrographic microscope, equipped with the usual 

 Bertrand lens and a micrometer eye-piece, is a device for measuring 

 the angle of inclination of an incident ray (as in the measurement of 

 of the optic axial angle); and, furthermore, that its sensitiveness to 



