342 MR. A. E. TUTTON OX A COMPENSATED INTERFERENCE DILATOMETER. 



one of the screws of the tripod is manipulated, and if the tripod has been conveniently 

 placed so that one screw is to the left side, in the diameter of the chamber floor at 

 right angles to the imaginary line at this height joining the pedestals of the 

 expansion apparatus and the telescope, the screw to rotate may be either the front or 

 back one, at pleasure. It then only remains to manipulate the screw to the left, in 

 order to bring the object image so as to more or less cover the adjusted image, 

 according to the size of signal stop employed and the width of interference band 

 desired. If the two images were absolutely coincident, the conditions would be those 

 for the production of alternate fields of light and darkness, rather than bands, during 

 alteration of temperature and the resultant alteration of the thickness of the air space 

 between the two reflecting surfaces, which, under these conditions, would be precisely 

 parallel. As parallelism is more and more deviated from, by manipulating the left 

 screw so as to bring the object image to cover less and less of the cover-wedge image, 

 the conditions are those for the production of bands, at first few and very wide, but 

 becoming narrower and narrower, and proportionately more numerous, as the process 

 of uncovering the first image proceeds. If an object were supported on the tripod, 

 and the air wedge consequently thin, the bands would be immediately visible if the 

 iris diaphragm were so far closed as to exclude all images but the double one, the 

 simple eyepiece replaced by the micrometer combination, and the white source of light 

 exchanged for a sodium flame. 



In order to employ mercury light for the determination of the expansion of the 

 screws of the tripod, or as in ordinary determinations, red or greenish blue hydrogen 

 light, the single reflecting prism of the expansion apparatus is replaced by the pair of 

 refracting prisms. These have previously been set for minimum deviation for the 

 green mercury line, which occupies a convenient mean position in the spectrum, with 

 the aid of the special slit signal stop. The source of white light is removed, and the 

 Geissler tube and its fitting attached, and connected to the induction coil. The 

 telescope is raised to the height of the upper refracting prism, and the induction coil 

 set in action. Instead of the four simple images, the right top one a double one, 

 there are now visible four corresponding vertical spectra, each showing sharp 

 brilliant images of the signal stop in the red and greenish blue, corresponding to 

 C and F hydrogen light, and a fainter one, so long as the tube is at the ordinary 

 temperature, in the bright green, corresponding to the green mercury line. The 

 reason for arranging the white light images as a rhombus, rather than as a square, is 

 now apparent ; for if the double image in white light had another image vertically 

 below it in the field, in the Geissler tube light the spectrum of the lower image would 

 partially overlap the double one due to the surfaces whose reflections are to produce 

 bands, and the interference bands would be blurred by the illumination due to the 

 undesired radiations. The angle of the rhombus is such that when the iris is arranged 

 no as just to permit of the complete passage of the desired double image in any of the 

 three colours, the other spectrum is entirely excluded. 



