14 DE. A. E. H. TUTTON: 



has been furnished with a black-glass fitting on each side, which not only renders it 

 symmetrical and evenly balanced on the two sides of the tube, but makes provision 

 for use of the interferometer on either side of either telescope. 



The pair of colourless-glass wedge-discs of 35 minutes angle, g l and g 2 , 5 cm. in 

 diameter and 1 cm. thick, referred to under (4), are mounted in a bracket fitting, G, 

 which is capable of sliding along the same dovetailed grooves in the front of the 

 upper bed as the prism-circle fitting. The fitting is provided with the means of 

 rigid fixation by a lever-handled screw, /, at the position which brings the glass 

 surface of <j. 2 nearest the microscope within a millimetre of the surface of the black- 

 glass disc, r/ 3 , carried by the latter. The wedge in question and its countervailing 

 duplicate, i^, are rigidly held in mounts against which they are pressed in each case 

 by a spring, whose force is almost entirely exerted against the cylindrical surface, 

 the normal (uubevelled) edge of the very thick disc, and produces absolutely no 

 bending of the plate, as proved by the perfect linear straightness of the interference 

 bands ; and each mount is held in position, one on either side of the annular termina- 

 tion of the fitting G, by three screws and springs, and can be separately adjusted by 

 means of these three screws. All six of the milled heads of these screws are on the 

 outer side of the fitting away from the microscope, and are thus readily manipulated, 

 those ft of the principal wedge-disc, </ 2 , next the microscope being lacquered bright 

 yellow as usual, and those k of the countervailing wedge-disc, <j lt lacquered black for 

 the sake of ready distinction. It is by use of the three bright screws, readily visible 

 in the darkened room, that the interference bands are adjusted, the black-glass disc, 

 y : ,, carried by the microscope being never touched after the initial adjustment to the 

 vertical plane. 



The Microscopes. 



The two microscopes are a duplicate pair which have been specially designed by 

 Messrs. K. and .1. Beck with the object of ensuring the greatest possible rigidity and 

 freedom from twist or bending of any kind. They are mounted symmetrically at the 

 inner ends (in order to enable them to be brought as near together as possible) of 

 their respective fine-movement plates carried by the sliding blocks, and counterpoised 

 thereon. The microscope itself is actually fixed to the front of a carrier bracket, cr 

 (figs. 3 and 4), in each case, which is screwed down on the top of the thick 

 fine-adjustment plane, n; the mounting is effected in a manner which permits of 

 adjustment of the optical axis of the microscope precisely to the vertical position, at 

 right angles to the horizontal axis of the fine-movement screw which causes the 

 traverse of the microscope. The counterpoise is a cylindrical leaden weight, m l and 

 m., adjusted on an arm projecting from the back of the bracket as much as the 

 microscope projects in front. When they are approached as near as possible to each 

 other, the optical axes of the two microscopes are slightly under 4 inches apart, just 

 adequate, in fact, to enable the end marks on a decimetre bar to be respectively 



