RECORDS. 185 



Summary of Papers. 



Mr. von Nardroff stated that this method was planned to 

 avoid the use of compensation, which leads to grave errors un- 

 less in the compensating material the ratio of the velocities for 

 any two wave-lengths is the same as in the substance being 

 measured. It is frequently impracticable to fulfil this condition, 

 as for example by using as a compensator a second plate of the 

 same material. Air compensation is of course out of the question. 



In the present method, in which no use is made of white 

 light fringes, the transparent plate, a microscope cover-glass, 

 for instance, is mounted on a special stage perpendicular to the 

 path of one of the beams in a Michelson interferometer. With 

 sodium light, bands are seen that are generally distorted through 

 lack of perfect parallelism between the surfaces of the plate. 

 The stage is now rotated forward about a vertical axis through 

 an angle of 45° up to a fixed stop, thus increasing the path 

 through the plate. Slowly turning the stage backward, the 

 bands passing a fixed point in the field are carefully counted 

 until the plate returns to the perpendicular position, when the 

 motion of the bands reverses. A new count is now made 

 while the stage is turned past the perpendicular, backward 45 

 degrees to a second fixed stop. Generally these counts differ 

 by a few tenths of a band, owing to imperfect mounting of the 

 stage as a whole on the interferometer, but they may be aver- 

 aged without sensible error. Since the light passes through 

 the plate twice, one half the number of bands counted should 

 be taken to represent the increase of optical path, ^V, in wave- 

 lengths. The thickness, /, of the plate at the part of it observed 

 in the interferometer may be measured by means of a microm- 

 eter caliper or by a spherometer. The following exact formula, 

 much simplified through the use of precisely 45 degrees of 

 rotation, gives the value of the refractive index, /i : 





