278 SCIENCE PROGRESS 



employed. When a hollow prism of glass, the faces of which 

 are plane and parallel, is filled with air at the same temperature 

 and pressure as that by which it is surrounded, the light which 

 passes through the prism suffers no deflection. But if some 

 other gas is substituted for the air, the light is refracted, and by 

 measuring the angle of the prism and the temperature and 

 the pressure of the gas its index can be calculated relatively to 

 that of air. The deflection is, however, very small. Thus, 

 in a prism of angle 143 , which is about as large as is convenient 

 (at higher angles it is difficult to obtain glass sufficiently well 

 worked to give a clear image), an atmosphere of air would 

 cause a deflection of 6' ; and in order to measure the index 

 to one part in a thousand it would be necessary to read the 

 position of the object through the prism to one-third of a second 

 of arc. 



The difficulties of the prism method depend on this fact. If the 

 faces of the prism are not absolutely plane and parallel, there is a 

 deviation due to this cause, which varies with the temperature 

 of the glass. It is also necessary to know the temperature, 

 pressure, and refractive index of the air which surrounds the 

 prism accurately, and the stability of the optical arrangements 

 must be of a high order. Owing to these difficulties the prism 

 method, which was used by Biot and Arago, by Dulong and by 

 Le Roux, was abandoned by Mascart after trial, and has not been 

 used recently in any great research, except that of Kayser and 

 Runge on the dispersion of air in 1893. In its place a method 

 founded on the interference of light waves has been adopted. 

 There are many varieties of apparatus, but they are nearly all 

 based on the same principle. A beam of monochromatic light is 

 divided into two portions, which are subsequently reunited in 

 the field of a telescope. If the path of one of these two beams 

 is retarded relatively to the other by half a wave length, the two 

 beams interfere and no light is seen. When the retardation is 

 an exact multiple of a wave length, the two reinforce one another 

 and the brightness of the field is increased. If, then, a source of 

 monochromatic light is viewed through such an arrangement, 

 the result is that in the field of view of the telescope is seen a 

 number of bright and dark bands which move across the field as 

 the path of one of the interfering beams is continuously retarded, 

 at the rate of one band for each additional wave length of 

 retardation. In practice, the two beams pass through different 



