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A NEW LOW-POWER CONDENSER. 



By Edward M. Nelson, FJR.M.S. 

 {Read April 28a, 1914). 



Fig. 4. 



Some time ago 1 pointed out to the Club that microscopists were 

 badly off for a low-power condenser, for, so far as I know, there 

 is no such appliance to be had. Mr. Curties kindly exhibits- 

 to-night one he has made from my formula. This condenser is 

 designed as a low-power illuminator, and not at all for the 

 purpose of resolving fine diatom striae. With the top on, its 

 focus is 1 inch, and with the top off 2 inches.* Both the lenses 

 are achromatised, and it will be seen that it is particularly 

 achromatic, as well as aplanatic ; it will work from the lowest 

 powers up to a | inch. 



The first object I examined with it was a Navicula lyra, with 

 a Zeiss 12 mm. apochromat. I have been working with the 

 microscope now upwards of forty years, and never before have I 

 seen such a perfect image of this diatom. In general work, 

 with the lower powers, the flat of the flame of a reading 

 lamp is focused upon the object ; this with the 2 inch condenser 

 covers a large portion of the field, even of the lowest powers. It 

 will give an excellent dark-ground for pond life, etc., up to say a 

 | inch objective. This condenser is to be named " Quekett," 

 after that illustrious microscopist. 



Speaking of dark backgrounds, there is a great defect in many 

 condensers, viz. that the spot is not centred to the optic axis of 

 the condenser, because the cell holding the stops is not placed 

 accurately on the mount. This is a serious defect, because if the 

 stop is not centred, the microscopist is forced to use a much larger 



* This back lens of 2-inch focus when used by itself in a holder forms 

 the best "verant" I have seen. It is very useful for the examination 

 of large microscopical objects, as well as of flowers, engravings, coin-, 

 postage stamps, seals, etc. 



