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Transactions of the Society. 



by patent ; but these I need not particularize, and will only briefly 

 describe the Microscope as constructed by them. 



I had neither time nor inclination to study any false " origi- 

 nality," or which served no useful purpose. Finding therefore the 

 general mechanical arrangement of the Duboscq Microscope, so far 



Fig 



as regards the rack-bar for the coarse adjustment, and the internal 

 rack-and-pinion movement for focusing the substage condensers, 

 well adapted to all my purposes, I adopted it. The action of the 

 French focusing rack, so awkward to English hands, was however 

 reversed to the usual direction by Messrs. Newton, and the useless 

 BO-called fine adjustment replaced by a steady screw movement on 

 the plan now so usual in histological stands. Optically the 

 arrangement is far different. In the Duboscq instrument one 

 small lens permanently fixed in the rack-tube is inteuded to 

 (and with the heliostat does) produce various kinds and angles of 

 illumimiting pencil ; whereas in mine various removable condensers 

 are employed, suitable for their purposes, and always before the 

 lantern focus is reached, except in the special case of an achro- 

 matic condenser presently described. 



I prefer to use a special triple lantern condenser of high angle 

 (95° or lUO^) 5 in. in diameter, which is calculated to utilize 

 the whole of the rays. In the nozzle of an ordinary good lantern 

 the illumination is somewhat less ; but as at the higher angle 

 there is more loss by reflection, and by the third lens, there is not 

 so much less as might be expected — not more in my opinion than 

 about 15 per cent. A special double condenser can be made 

 nearly equal to the triple form. 



