16 DE. A. E. H. TUTTON: 



Their actual appearance as employed by the author will be seen in fig. 1 3 (Appendix), 

 which also shows the spider-lines of the micrometer (the three single lines, in the 

 figure). 



The f-inch objectives are specially designed to focus down into the ^-inch wells in 

 the standard yard bars, at the bottom of each of which a fiducial mark or defining 

 line on a gold plug is situated, without any contact of the objective with the top of 

 the bar ; that is, the working distance is just over half an inch. The two objectives 

 of each microscope can be readily interchanged, being screwed not directly into the 

 microscope tube but into a dovetailed slider, which only requires to be pushed home 

 into corresponding grooves in a frame, f, carried at the lower end of the microscope 

 tube, until flush with the front of the frame. The slider may then be tightly locked 

 in position by rotating for 180-degrees a little lever pivoted to the frame. Imme- 

 diately above this terminal frame each microscope tube carries a transparent, thin, 

 and truly plane glass reflector, for the illumination of the object (the rulings or 

 defining line) through the object-glass. The reflector is rotatable about a horizontal 

 diametral axis, parallel to the length of the bar ; and opposite the reflector the tube 

 is pierced by two windows, through one of which (the back one) it can be illuminated 

 by the source of light to 1)6 employed. Provision is made for the adjustment of the 

 reflector by rotation in the horizontal plane, the whole of this lower portion of the 

 tube, K, being rotatable in a very stiff manner about the upper portion, as it is 

 carried by an inner tube rotating, without sliding, very tightly in the main microscope 

 tube. 



The source of illuminating light for all preliminary work is a miniature electric 

 lamp, E, with a short but thick rectilinear filament, an image of which can be thrown 

 directly across the standard bar at the spot where the defining line or Graysou ruling 

 is situated, which can thus be illuminated throughout its length. The suspension of 

 the lamp, in the space between the microscope tube and the front of the slider-block 

 and upper bed, which are both vertically flush with each other, permits of adjustment 

 for height and for lateral position, the latter movement being provided with a fine- 

 adjustment screw, S. Between the lamp and the back window of the tube opposite 

 the reflector is the short tube, T, carried by an arm projecting from the lamp carrier, 

 and provided with a collimating lens at the lamp end and an adjustable iris diaphragm 

 at the reflector end. These together enable precisely that cone of illuminating light 

 to be produced which is most favourable for the clear focussing of the particular mark 

 which is the object of observation, a great advantage when the latter is a fine Grayson 

 ruling, the thickness of each line of which is less than half a wave-length of red light. 

 By means of the collimating lens the image of the short filament is projected back so 

 that a virtual image is formed at a distance from the object-glass equal to that of the 

 back focus of the eye-piece, and the iris diaphragm controls the angle of the illumi- 

 nation, thus ensuring so-called " critical" illumination for opaque work, and, indeed, 

 as accurately as is given with a sub-stage condenser for transparent objects, the object- 



