MB. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 329 



edges of the aperture and corresponding bevels on the slider, the lower horizontal end 

 edge of which forms the upper jaw of the slit. Both this edge and that of the main 

 strip which forms the lower end of the aperture and acts as the lower jaw of the slit, 

 are bevelled. The top of the movable slider projecting above the handle is furnished 

 with a smaller handle, for convenience in setting it so as to adjust the fineness of the 

 slit. This slit enables the spectrum of the illuminating source of light to be studied. 

 As the light traverses the two refracting prisms twice, the apparatus aftbrds a disper- 

 sion equal to a spectroscope of four such prisms, and hence the study of the spectrum 

 is highly interesting. 



The iris diaphragm, f, is placed almost in contact with the fixed diaphragm, r/ ; 

 it is manipulated from outside by means of a lever handle traversing a slot, upon the 

 edge of which is a graduated silver arc, the indications of which, corresponding to 

 definite apertures, are recorded by an indicator attached to the handle. 



The lens of the illuminating tube is carried in a short sliding tube, t, so that the 

 most favourable position for the illumination of the rectangular signal aperture can 

 be attained. When white light is being employed for adjusting purposes, or when 

 sodium light is being used for observations of the bands, this is all that is necessary. 

 When a capillary Geissler tube for longitudinal vision is being employed as source 

 of light, a further arrangement is needful for adjusting the capillary so as to attain 

 the maximum illumination of the signal. The Geissler tube employed is of the 

 H type recommended by PULFRICH, which consists of two wide terminal tubes, >i, 

 arranged vertically and fitted with aluminium spiral terminals, and a horizontal 

 capillary connecting tube, v, the brilliant glow in which, regarded end on, is the 

 source of light. The tubes, after Dr. EIEDEL'S pattern, were supplied by ZEISS. 

 They contain a hydrogen vacuum, and a globule of mercury in one of the wide 

 limbs, which is placed furthest from the lens, and on gently warming which the green 

 light due to mercury vapour, corresponding to the green line of the mercury spectrum, 

 makes its appearance. This particular radiation, when separated from the hydrogen 

 radiations by the train of prisms of the expansion apparatus, forms the most 

 effective monochromatic source of illumination for the production of interference 

 bands when the two reflecting surfaces are at a considerable distance. 



The supporting and adjusting arrangement for the Geissler tube affords the means 

 of both centering and adjusting, so as to bring the capillary exactly into the pro- 

 longation of the optical axis of the illumination tube, and also for moving it 

 longitudinally in this axis to the most suitable distance from the lens. It consists 

 of a tube, iv, which, fits fairly tightly over the broad flange which forms the lens 

 mount of the sliding tube ; this flange is of slightly greater diameter than the 

 illuminating tube itself, hence the latter is not injured by the placing in position and 

 removal of this attachment. On the outside of this attached tube slides another, x, 

 movable over it by means of a pinion gear carried at the end of the tube nearest 

 the observer and travelling with it, which gears with a rack carried by the inner 

 VOL. oxci. A. 2 u 



