MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 341 



The heating bath had been temporarily removed. The telescope was fitted with the 

 simple eyepiece and was arranged for parallel rays, the eyepiece focussing the vertical 

 line of division of the iris aperture formed by the edge of the little reflecting prism. 

 An incandescent electric lamp with ground glass globe, and provided with an opaque 

 shade pierced by a circular aperture the same size as the illuminating lens, was 

 brought close to the end of the side illuminating tube, the Geissler tube fitting 

 having been removed. The telescope was then directed at the reflecting prism of 

 the expansion apparatus, and the prism of the latter adjusted so that the images of 

 the rectangular signal stop in front of the small reflecting prism of the telescope, 

 reflected from the four glass surfaces of the two wedges and from the platinum- 

 iridium table, could be viewed and any one brought into the centre of the clear 

 half of the iris aperture by slight movement of the telescope. The iris aperture 

 should be open to the full during this operation. By use of the rack and pinion of 

 the telescope any slight want of sharpness of the images can immediately be 

 corrected. 



There are, in general, five such images from the interference chamber, two due to 

 the two surfaces of the upper wedge, two to the cover- wedge, and one from the object, 

 in this case the platinum-indium table. They are easily recognisable. By slightly 

 lifting and replacing the cover-wedge, the two due to that are distinguished by their 

 temporary movement or disappearance. The angle of the wedge is such that the two 

 images are so far apart in a horizontal line as to be just incapable of being simul- 

 taneously visible in the semicircular field of view. The pair of images due to the 

 upper correcting wedge ave equally distant from each other, but they should be 

 about the same distance below the other pair as they are apart from each other, and 

 a little to one side, owing to the suitable tilt given to the wedge by the two small 

 tilting screws. These four images are thus arranged at the four corners of a 

 rhombus. 



The first thing is to distinguish which of the two cover-wedge images is afforded 

 by the lower surface. The author always works with the thickest side of the wedge 

 to the right, and this causes the image from the lower surface to appear to the right 

 of the other. Hence, the image in question forms the right top corner of the rhombus. 

 Moreover, if the lengths of the platinum-iridium screws had been adjusted to 

 approximate equality, the image due to the object or tripod table would lie very near 

 to this. The next thing is to adjust the image from the lower surface of the cover- 

 wedge so as to lie symmetrically to the horizontal diameter of the field, and almost in 

 contact with the vertical edge which divides the light half from the dark half of the 

 field. This is achieved by movement of the telescope by the fine azimuth adjustment 

 screw and the tilting screw. The next operation is to bring the object image exactly 

 to the same height as the one just adjusted, that is, also symmetrical to the horizontal 

 diameter. If this is not done exactly, the interference bands will be oblique to the 

 vertical diameter of the field instead of parallel thereto, as is desired. To achieve it, 



