26 DR. A. E. H. TUTTON: 



sentation of the appearance while the front lens-combination is removed. All but the 

 second spectrum (the double one corresponding to the images A and B) can just be 

 excluded, without impairing the illumination of the double red image required and 

 leaving that alone in the centre of the field ; the 1st and 3rd spectra are just out oi 

 the field and the 4th considerably so. The very faint traces of continuous spectra of 

 A and B are negligible compared with the brilliance of the sharp A and B images in 

 Ha red and are quite without effect on the bands. A little adjustment may possibly 

 be required to bring the double image in red light (or greenish-blue if the prism has 

 been adjusted for F-light) to occupy exactly the position shown in fig. 8, which is, at 

 the same time, vertically central and quite close to the sharply focussed vertical edge 

 of the semicircular aperture. This is effected, if needed, by slight altitude and 

 azimuth adjustment of the telescope, supposing the prism to have been correctly 

 adjusted at the proper circle reading for the radiation in question, from the knowledge 

 acquired in the preliminary work. On now replacing the front-lens combination, the 

 field of interference bands will be seen in the colour for which the prism has been 

 adjusted. 



Its correct appearance is clearly shown in fig. 6. The comb, or rough scale, 

 should be arranged as shown, with the circular image of the black-glass disc g a 

 in the red or green light, for which the adjustment has been made, fully exposed. 

 The silvered ring in the centre of the surface of the colourless plate <J 2 , nearest to g s , 

 should be sharply focussed if the special single lens of the eye-piece is arranged in the 

 tube of the correct length for this working distance, as already ascertained by 

 preliminary work. If then, however, the focus is not quite sharp, it is because the 

 telescope requires a little adjustment in the direction of its own length, by means of 

 the front wheel, that corresponding to the front-and-back movement /8, of the adjusting 

 movements at the base of the pedestal. When the reference ring is sharply focussed, 

 the bands are thereby automatically focussed, as they are actually produced between 

 the two surfaces, y. 2 and i/ 3 , which are less than a millimetre apart, and one of which, 

 </ 2 , actually bears the ring. 



The bands may not be quite of the desired width, and they will probably also be 

 very slightly inclined to the vertical, as the adjustment of the A and B images cannot 

 be done to the required fineness to effect these adjustments absolutely. These defects, 

 however, can both be simultaneously remedied by careful slight manipulation of one 

 or possibly two of the three bright screws h which adjust g 2 , these adjustments being 

 thus more delicately performed on the bands themselves. 



The spider-lines then require to be set at the requisite distance apart by means of 

 the left drum of the micrometer, so that any one band may be clearly adjusted between 

 them as in fig. 6 ; when the proper width of band is attained, this adjustment of the 

 separation of the parallel spider-lines is such, as brings them just within the little 

 silvered ring, a small segment of which should show equally on each side, as also shown 

 in fig. 6. Everything is then adjusted concerning the interference bands. 



