98 Improvements in the Microscope. [*!,°o"umL. "IbTma' 



distance from eacli other, gives a focal distance of IS™'", and so the 

 second magnifying is = 9 This apparatus having the objective 

 and middle eye-piece combined with the five eye-pieces of Hartnack 

 (magnifying fi-om i^ths and xoths to 11 diameters), gives a total 

 power of from 6840 to 19,800 diameters, with a tube of 440'^'". 



Professor Listing advises that the lenses of the eye-piece should 

 be made of 15™°^ diameter, and with a correction for their distance. 

 For the middle eye-piece, perhaps, lenses of quartz combined with 

 a lower (1"61 to 1-59) flint glass should be used. In another 

 place he gives a different construction for the middle eye-piece, 

 analogous to an objective of two glasses, but with greater dimen- 

 sions, and calculates the magnifying power of this to be from 

 22,000 to as much as 25,600 diameters. 



Professor Listing observes that only the penetrating power 

 would be raised by this method of construction, but that to a 

 very considerable degree. — A paper read before the Microscopical 

 Section of the Boston Society of Natural Science, November 10, 

 1869. 



