,; DR. A. E. H. TUTTON : 



with the latter on a divided circle, / (iii) An interference apparatus, g, consisting 

 essentially of three truly plane glass plates, whose axial line normal to the surfaces is 

 horizontal. Two of them, f/i and g 2 , of colourless glass, are separately referred to 

 under (4), and the third, g 3 , of black glass, is carried by the right-hand microscope ot 

 the two referred to under (3). All the refracting and reflecting glass surfaces of this 

 optical train are thus vertical, instead of horizontal as in the dilatometer and elasmo- 

 meter. All are mounted rigidly on (2), except the black-glass disc. 



(2) A V -and-plane bed, h. This is constructed of specially chosen cast iron, the 

 casting of which was most carefully carried out. It is 6 feet 6 inches long, and rests 

 in a plinth, *', of the same metal, bolted down to the upper surface of a stone block, k, 

 with special precautions to avoid strain. The latter, the rigid support for the whole 

 instrument, except the interferometer telescope, is 7 feet 6 inches long, 4 feet 4 inches 

 high above the floor of the room, and 2 feet 2 inches thick at the base ; it is vertically 

 upright in front and at the ends, but tapers towards the top at the back of the block 

 away from the observer. This large block of stone rests on a concrete basal continua- 

 tion, which is carried right down 4 feet below the floor to the sandy loam foundations, 

 with an air space of 6 inches all round, as shown in fig. 5, which gives the foundation 

 plan and two elevations as kindly supplied by H.M. Office of Works. The front 

 facial edges of the cast-iron bed are grooved out to form a dove-tail, /, to take the 

 prism-circle fitting and the supports of the glass discs referred to in (l, ii and iii). 



(I) A pair of similar microscopes, M. l and M 2 . These are arranged vertically and 

 at the same height, with truly parallel axes, and each is mounted on a relatively very 

 thick fine-movement sliding plate, n, of the same cast iron as the bed, 10 inches long, 

 4f inches wide, and 1 inch thick. This plate, or slab, in each case moves by manipula- 

 tion of the all-important fine screw referred to in the introduction, with V-and-plane 

 contact, over a similar thicker block, o, but, for sake of smooth sliding, constructed ot 

 steel, 11 inches long and 3 inches thick. This block slides on the cast-iron V-and-plane 

 bed, //, referred to in (2). The right-hand microscope, M 2 , carries the black-glass 

 disc, <y s , of the interference apparatus referred to in (1, iii), whose outer surface is one 

 of the two truly plane-polished vertical surfaces which reflects the interfering light 

 so as to produce bands which art; visible through the interferometer telescope. The 

 fine, screw is of -^-inch (0'5-mm.) pitch, and bears a large silvered drum-head, p, 

 graduated directly into 1000 parts by the method already described in connection 

 with the author's elasinometer. It also carries both a milled head, q, for the direct 

 rotation of the screw, and a fine-adjustment worm-wheel, r, of 100 teeth gearing with 

 an endless screw, s, manipulated by the control wheel referred to under (7). 



(4) A pair of similar colourless-glass discs, g l and g 3 , part of the interference 

 apparatus referred to under (1, iii), 5 cm. diameter and 1 cm. thick. The two surfaces 

 of each disc are polished absolutely truly plane, but are not strictly parallel, being 

 35 minutes inclined to each other, which is just adequate to laterally displace the 

 reflection from one surface when that from the other is in the centre of the field of 



