136 Transactions of the Society. 



•j'g-in. objective is used for practically all the finer work, to centre 

 all the condensers to the T^^-in. objective in use. « 



Under this changing device would be the iris diaphragm and 

 stop carrier, which would be of the Abbe design, with the addition 

 of an extra turn-out ring to carry screens. On many instruments 

 it is impossible to use a stop and colour screen simultaneously in 

 the substage. For those who use two condensers only and desire a 

 less revolutionary change, I suggest that the substiige be provided 

 with two rings mounted on hinges some distance either side of the 

 centre line, and each provided with its own centreing gear. Either 

 of these could be in turn swung into position, and held by a catch 

 against a strong lug on the substage rack slide. By this means the 

 necessit}'' of removing the condenser and re-centreing when changing 

 from normal to dark ground illumination would be obviated. In 

 designing centreing gear for the ordinary substage ring, I would 

 point out that the pattern in which the moving ring is pressed 

 against the flat upper surface of the outer ring by the centreing 

 screws and buffer, which act on steel-faced inclined planes, is much 

 to be preferred, on mechanical and practical grounds, to the system 

 employed by most English houses. The addition of a locking nut 

 to the spring buffer which opposes the centreing screws would be 

 of value and increase the rigidity of the condenser fitting. 



« 

 Substage Condenser Fittings. 



Complaints are frequently made regarding the faulty placing 

 of the iris diaphragm and stop carrier in modern English condensers 



EXPLANATION OF TEXT-FIGURE. 



Showing structure of rotating centreing stage in three radial sections : 

 I. Through centreing screw. II. Through buffer: III. Through 

 clamp screw. 



X-Y. Centre of stage and optic axis. 



A. Bottom rotating plate of stage, bearing prismatic slide H for mechanical 



stage. 



B. Bronze ring with channel of triangular section turned in its periphery. 



C. Steel bearing-ball. 



D. Bronze end to centreing screw with conical hollow in head. 



E. Centreing screw. 



F. Fixed base of stage, attached to limb. 



G. Milled head to centreing screw. 



H. Prismatic bearing for mechanical movements. 

 K. Pin of spring buffer. 

 L. Index to record degrees of rotation. 

 M. Rib on K to prevent rotation of L. 

 N. Spring of buffer. 

 0. Clamp screw engaging in R. 

 P. Milled head to clamp screw. 



R. Serration in bottom of channel to give clamp screw a positive locking 

 action. 



Diagram drawn to a scale of x 2. 



