12 DE. A. E. H. TUTTON: 



prisms, it is adequate for the purpose of separating and isolating the images of the 

 rectangular stop of the little auto-collimating reflecting prism of the telescope (this 

 stop being considered as the origin of the interfering light), corresponding to the three 

 different hydrogen lines or to the various cadmium or mercury lines. It affords a 

 much readier means of changing from one wave-length to another, mere rotation of 

 the circle carrying the prism being alone necessary, no movement of the telescope at 

 all being required. It is only necessary (a) to move the front lens-combination, which 

 focusses the spider-lines, from the front of the eye-piece, when the brilliant image of 

 the little rectangular stop is seen in the colour for which the bands are adjusted, red 

 C-hydrogen light, for instance, assuming a hydrogen vacuum tube is being used, 

 together with a faint continuous spectrum ; and (b) to rotate the prism circle so that the 

 faint spectrum moves in the direction of the wave-length to which it is desired to 

 change that of F-hydrogen light for example until the brilliant image of the stop 

 in the colour desired (greenish blue in our example) is brought into position in the 

 semi-circular aperture instead of the former image. On replacing the front lens- 

 combination, the field of bands will be seen in the colour desired. The bands are, of 

 course, closer together for F-hydrogen light than for C-hydrogen light, on account of 

 the shorter wave-length. If we again remove the front part of the eye-piece and go 

 on further until the Hy-violet image is similarly adjusted, on replacing the lens- 

 combination the field of bands in violet hydrogen light will be seen, quite clear and 

 sharp, although, as might be expected, not so brilliant. 



The constant-deviation prism is carried on a rotating divided circle, f, which is 

 carried by a fitting, 8, capable of sliding along the front vertical face of the upper bed 

 in a pair of dovetailed grooves, and of being fixed in any desired position by means of 

 a convenient tightening screw manipulated by a lever, e. The circle is divided into 

 half degrees, and reads with the aid of a vernier to minutes. The cone within which 

 it rotates is borne by a stout arm, 77, radiating from a vertical slider, 0, in the main 

 fitting, S, which is adjustable for height by means of a milled-headed screw, A. (which 

 drives the slider), at the top of the fitting ; it is also adjustable, as regards its 

 ' back-to-front distance from the face of the bed, with the aid of the horizontal 

 movement of a slider, /A, carrying the cone along the arm, brought about by means of 

 a milled-headed screw, IT, at the extremity of the arm. Thus the sliding motion along 

 the bed front gives one lateral adjustment to the prism circle and the prism which it 

 carries; the back-to-front movement affords the second rectangular horizontal 



O 



adjusting movement, and the up-and-down motion provides the third necessary 

 adjustment for the prism. 



Besides the constant-deviation prism, there are provided two 60-degree prisms for 

 alternative use as in the author's interferometer, and these and the constant-deviation 

 prism are separately mounted and adjusted, each by means of its own tripod screws, 

 on a carrier table, p, which drops automatically into position on the circle plate by 

 three pins fitting in corresponding holes in the plate. A third carrier bears an adjusted 



