110 C. Barns — Displacement Interferometer. 



In view of these advantages among others, I have con- 

 structed a definite form of apparatus for displacement inter- 

 ferometry, specially adapted for general observations, such for 

 instance as I have in view with fog particles. The apparatus 

 is to be light, portable, rigid, with relatively long distance 

 between the opaque orthogonal mirrors M and JV, and the 

 oblique mirror or grating, as well as height of mirrors above 

 the arms, and with an easy adjustment for different angles of 

 incidence /, large and small. In the following apparatus, figs. 

 1, 2, and 3, the distance between the center and either remote 

 mirror is about 35 cm . It may easily be enlarged many times. 

 The angles of incidence /= 15°, 45°, and 75°, are available at 

 once for the given braces, though of course other angles may 

 be used. 



The long arms and feet of the apparatus, which in general 

 form is naturally much like a spectrometer, are made of 1/4 

 inch gas pipe, and the braces are heavy strips of tin plate, bent 

 so as to be U-shaped in cross section, much like umbrella 

 steel, with the ends bolted down. In the drawing (of which 

 fig. 1 is the plan and fig. 2 the elevation) the axles are cylindric 

 or slightly conical. In my own apparatus sufficient rotation, 

 180°, of the parts was secured by ordinary well-cut gas pipe 

 screws. The long arms of gas pipe a, c, d, e are not only con- 

 venient for the attachment of objects to be examined, by ordi- 

 nary clamps, but they admit of a circulation of cold water, so 

 that their lengths remain invariable whatever be the tempera- 

 ture of the environment of air. 



The tripod, figs. 1 and 2, carries a standard Q of 3/8 inch 

 gas pipe, which is secured snugly by the cross-coupling P. 

 From this the horizontal rigid arms, a and <?, lead respectively 

 to the collimator A and to the slide micrometer O and they are 

 screwed into P parallel to the plane of fig. 1. The arm e 

 which carries the telescope E must be revolvable around Q, 

 a wide axle PP' and braces V diverging as they approach Q 

 sufficing for the purpose. The telescope is used for reading 

 only, and need not be clamped. It must, however, be quite 

 firm so as not to shake the instrument. 



The standard Q is prolonged above the cross-coupling R as 

 shown at j and the graduated plate at K (for measuring the 

 angle of incidence /) rotates around Q prolonged. The plate 

 may be clamped by the set screw j^. Radially to _K, the lat- 

 eral arm d is bolted below the plate at f. D carries the 

 opaque mirror J/, which thus rotates around Q within 



The adjustment chosen is such that the parts M, JV, G (the 

 grating), the telescope E and the collimator A may be dis- 

 placed upward several inches, in the clear. It is thus possible, 

 for instance, to place the fog chamber between M G or NG. 



