102 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



with an iris diaphragm. The stage can be fixed at any angle by means 

 of a clamping handle, and the draw-tube is marked with millimetre 

 divisions. Instead of a rectangular, the instrument can be supplied with 

 a circular stage of 95 mm. diameter (fig. 20), the rotation axis of which 



Fig. 26. 



can be centred on the optic axis of the instrument by means of two bind- 

 ing screws. By means of these covers there is afforded a displacement 

 of 6 mm. in every direction, so that this stage may be made to work as a 

 travelling stage. 



Differential Screw Fine Adjustment.* — W. Forgan had three 

 " two speed " differential fine adjustments made upon Ashe's plan.|| In 

 the first, the quicker motion was 3^ inch, and the slower 5^5- inch for 

 each revolution ; in the second, the slowest motion was j^Vo' and in the 

 third, •j-J-j for a revolution. As some slight hesitancy was observed 



* Proc. Scot. Micr. Soc, 1903-4, p. 47. 



|| Journ. Quekett Micr. Club, ser. 2, viii. (1901) p. 131 ; Journ. R.M.S., 1902 

 p. 232, figs. 40-2. 



