MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILA.TOMETER. 343 



The telescope is now to be so arranged for height, tilt, and azimuth that the 

 double image in the desired colour is symmetrical to the horizontal diameter of the 

 field, and almost in contact with the vertical line of demarcation between the light 

 and dark halves of the field. The paths of the incident and reflected light are then 

 almost identical, and the condition for maximum brilliancy of bands is attained. The 

 micrometer combination, fitted with the movable lens carried in its shorter tube, is 

 now attached in place of the simple eyepiece, and its eyepiece is withdrawn, to 

 enable the observer to inspect the double image after the change and ascertain that 

 it is still in position. For it conveniently happens that when the eyepiece of the 

 micrometer is withdrawn, the movable lens left in position acts as a simple eyepiece, 

 although not quite so well as the simple eyepiece supplied for the purpose, being 

 further from the eye and not quite at the focus of the vertical edge of the semi- 

 circular field ; it affords, however, images which are sufficiently good for the purpose 

 of adjustment of their proper position with respect to the iris aperture, or of the 

 relative position of the two overlapping images required to produce the bands. 



On replacing the eyepiece in front of the micrometer, the interference bands are at 

 once seen in the colour adjusted for. If an object is being used, and the air wedge is 

 thin, the bands are exceedingly brilliant in red hydrogen C light, and only slightly 

 less so in greenish blue F hydrogen light. . In green mercury light they are faint at 

 the ordinary temperature of the Geissler tube, but most brilliant if the capillary and 

 the limb containing the mercury globule are gently warmed. The observer no 

 longer sees a semicircular field, as with the simple eyepiece, but a full circular field, 

 completely covered with bands if the object is as large as the diaphragm containing 

 the upper glass wedge of the interference chamber, The semicircular opening, 

 which is really very small when the iris is arranged as has been described, now 

 simply acts as a diaphragm to cut off undesired light. The disturbing reflection from 

 the inner surface of the objective, which would otherwise be very serious, is also 

 thrown behind either the iris diaphragm, or the micrometer serrated diaphragm, by 

 the device for slight tilting of the objective which has been described in the earlier 

 portion of the memoir (p. 328). 



The bands can be adjusted to the desired width by manipulation of the left-hand 

 screw of the tripod, and any inclination from the vertical, as observed by reference 

 to the pair of vertical spider -lines, can be corrected by slight rotation of the tripod, 

 or, if preferred, of the micrometer itself within its outer carrying tube. The most 

 suitable width is dependent upon the nature of the object. If its reflecting surface 

 is truly plane, as proved by the severe test imposed by its use for the purpose under 

 discussion, namely, by its generation of rectilinear bands at regular distances, the 

 width of band chosen may be more considerable than when the bands are slightly 

 curved or the distance is not quite regular, as the irregularities largely disappear 

 when the bands are closer together, produced by an air wedge of greater angle. 

 Usually a breadth of about 100 drum divisions, one complete rotation, is a con- 



