ADJOURNED DISCUSSION IN LONDON 



239 



The arrangement shown in the accompanying sketch was made 

 in the w^orks at very small cost, and has proved very satisfactory. 

 The short metal rod which fitted into the socket on the focussing rod 

 has been replaced by a longer rod, J in. square in cross section. 

 A IJ in, pulley wheel, turning on flanges, is mounted on a bracket 

 at the corner of the microscope base. A square hole through the 

 pulley wheel is just large enough to allow of very slight play between 

 the wheel and the square rod. As the square rod w^ill pass along 

 through the pulley wheel, horizontal traverse of the microscope stage 

 is not interfered with. A long rod is mounted in brackets screwed 



to the base, which carries the camera. This rod has a 2 iii- pulley 

 wheel at one end, which is connected by cord to the other wheel. 

 At the other end is a 2 in. milled brass head, for turning. The 

 two grooved wheels over which the cord passes are milled inside the 

 grooves, thereby preventing slip. The arrangement as made to 

 dimensions given above further reduces the speed of fine adjustment 

 by one half. The main advantage is that, owing to slight play 

 between wheel and square rod, any slight torsion produced whilst 

 turning the rod can ease itself when the hand is removed, without 

 turning the fine adjustment and disturbing the focus. 



Another matter, referred to by a previous speaker, is the lack 

 of contrast in most metallurgical specimens as compared with a 

 biological section. This is one of our difficulties, and, as a matter 

 of fact, most of our photomicrographs show considerably more con- 

 trast than is actually present in the specimen. 



Mr. T. Smith : I would like to have spoken on the optical side 

 ■of the discussion, but there is one matter I will refer to. We have 

 Iveen given some figures by Commander Ainslie based on a displace- 

 ment of 1/100 mm., and some further results on a basis of the same 

 magnitude, with the displacement along the axis, may be of interest. 



