18 DK. A. E. H. TUTTON: 



T-bolt carried below the slider as a stud, which enters the corresponding T-shaped 

 channel, 0, running through the whole length of the main bed, as described in the last 

 section ; the locking is effected by means of a lever, D, projecting behind from an 

 aperture cut out of the back part of the block, a plate spring being introduced to 

 prevent strain. The sliding of the block is done by hand as this is only the coarse 

 adjustment of the position of the microscope above the bar. Throughout the entire 

 length of each steel block a cylindrical hole lias been bored, and ground with great 

 care truly cylindrical and parallel to the length of the block and bed. Within this 

 fits a hard phosphor-bronze cylinder, with adequate nicety to allow free longitudinal 

 motion without the least shake, but keyed to prevent rotation. Into one end of this 

 cylinder is fitted a steel screw of 50 threads to the inch, the length of actual screw 

 thus threaded being 3 inches, affording ITT inches of fine movement on each side 

 of the normal position. It is maintained in place by a suitable bronze flange screwed 

 to one end of the steel block, and at the other end a strong flexible steel spring 

 is compressed between the cylinder and the plug which closes the boring, in order 

 to prevent backlash ; the spring opposes the screw when the latter is turned 

 anti-clockwise. Outside the bronze flange, on the portion of the steel screw which 

 comes through the latter, is fitted the worm-wheel, r, of 100 teeth, already referred 

 to under (3), which is engaged by the endless screw, s, also mentioned in (3), 

 fitted at right angles to it ; witli the aid of a spring, F, and cam, H, actuated by a 

 lever, K, it can immediately be thrown out of gear when desired, or as readily 

 brought into position. The endless screw has a milled head, L, for direct rotation, 

 and it is also continued with square section sufficiently far in front of the milled 

 head to gear with the shaft, y, of the large wheel-control, w, separately mounted 

 adjacent to the interferometer telescope. One revolution of the endless screw by the 

 milled head of the large wheel obviously advances the fine screw -g-^QQ of an inch or 

 O'OOo mm., which is equal to 15 interference bands of red light. 



Outside the worm-wheel the screw shaft carries the silvered drum, p, referred to in 

 (3), which is divided into 100 parts directly; by 100 oblique lines and 10 circles 

 parallel to the first directly divided one, a diagonal scale of 10 further sub-divisions 

 is produced, corresponding to thousandths of the pitch. Each oblique line passes 

 from a division on the directly divided circle on which the hundredths are numbered 

 to the next division on the tenth of the other circles,' which had been similarly and 

 exactly parallelwise divided, so that the obliquity is to the extent of a division. An 

 indicating straight line parallel to the axis is carried on a glass plate, mounted in a 

 frame, N, suspended by a bracket over the drum. This enables the thousandths to be 

 read with great ease. At the extreme outer end close up to the drum the shaft 

 terminates in the large milled head q, for direct hand rotation of the fine screw ; the 

 endless screw should obviously be thrown out of gear before this is used. 



In the centre of the top of this steel slider-block a recess is cut out, exposing the 

 phosphor-bronze cylindrical shaft for about 2| inches, and on this exposed portion of 



