NELSON DARK GROUND IMMERSION CONDENSER:

c. 1925

Maker: W. WATSON & SONS LTD.

Serial number:7097

About 1925, another dark ground condenser, designed by the eminent microscopist E.M. Nelson was released by Watson as Nelson's Cassegrain Dark Ground Illuminator. This condenser, which fits the same housing as the Holos Immersion Paraboloid, was also called Nelson's Reflex Condenser. This illuminator was designed for slides of 1 to 1.2 mm thickness. It was supplied with 2 different diameter stops, although only one labeled 16 is present with this example. This condenser has the advantage of allowing the full N.A. of oil immersion objectives to be used with N.A. up to 1.40. This type of high power dark ground condenser is much less complex and easier to manufacture than the cardioid type*, though the latter is usually optically superior. The sequence below shows the dissassembly of the condenser. With the top off for lower power dark ground work, and with the bottom lens removed revealing the stop, and finally with the stop removed from the housing.

*as found for example on Beck's Focussing Dark Ground Illuminator.