320 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



tube, and of correcting the slight prismatic deviation of the light rays introduced by 

 making the cover-wedge slightly wedge-shaped. The correction is achieved by 

 making the disc, e, equally wedge-shaped. In fact, the two discs are cut from the 

 same piece of plate glass, which has been ground down so that the two perfectly 

 plane surfaces are 35 minutes out of parallelism. The direction of the wedge is 

 marked by a straight line engraved parallel to the long edge of the slab before 

 cutting the discs from it. The latter are so cut that this engraved line comes near 

 the margin of each, forming a chord of the circle. It is only necessary to place the 

 two discs so that the engraved chords are parallel and on the same side of the centre, 

 but so that the marked surface of the larger disc of the interference apparatus lies 

 uppermost while that of the smaller diaphragm disc lies underneath, when the 

 refractive effect is fully counteracted. In order that no reflections shall be visible 

 due to the two surfaces of the countei-acting diaphragm wedge, the latter is slightly 

 tilted by means of two very small screws driven through the rabbet of the aperture 

 from the under side in such positions that the line joining them is parallel to the 

 engraved chord of the wedge ; when the disc is laid in its rabbeted aperture it 

 rests upon the two screw points and a third equi-distant point on the surface of the 

 rabbet, the lines joining the three points of contact thus forming an equilateral 

 triangle. The slight tilt given to the disc when the screws are properly adjusted is 

 sufficient to throw the reflected rays so far out of the optical axis of the system as 

 to be no longer visible through the micrometer eyepiece. 



The chamber terminates above the diaphragm roof in a screw thread, f, which 

 engages with one on the lower end of the supporting apparatus. Below, the 

 chamber ends in an adjustable table, g, upon which rests a disc of asbestos millboard, 

 which, in turn, supports the glass floor disc. The adjustment is effected by three 

 milled-headed screws, h, worked from underneath, which raise or lower the table 

 above the rigid gun -metal base, &, of the chamber through which the screws pass. 

 This rigid base is also perforated by a central hole, through which passes a short 

 cylindrical rod fixed to the under side of the adjustable table ; the rod terminates in 

 a boss, I, between which and the under side of the fixed base a spring is confined, 

 which causes the adjustable table to be always pulled firmly down upon the ends of 

 the three screws. The chamber can be closed when desirable, in a light-tight manner, 

 by a concentric outer tube, m, capable of sliding over it from above, with sufficient 

 friction to maintain it up when the chamber is required to be open during adjustment 

 of the interference apparatus. The author removes it altogether when the interfer- 

 ence chamber is immersed in the air bath for the purpose of observations at the 

 higher temperatures. The rigid base of the chamber and its adjustable table may 

 be readily detached from the chamber if required, but the windows are sufficiently 

 large to admit the tripod without this necessity arising merely for that purpose. To 

 provide for its occasional desirability, however, the lower end of the chamber wall 

 terminates in an annulus and flange, n, which rest upon a corresponding rabbet cut 



