Bicknell.] 



44 [October 12, 



The following paper was presented : — 



A Method of Producing very Low Powers for the 

 Microscope. By Edwin Bicknell. 



I use a plano-convex " collecting," or " reducing " lens, in the draw 

 tube of the microscope, about midway between the objective and eye- 

 piece; it is achromatic, of four inches focal length and six-tenths of 

 an inch in diameter. 



Placed about midway between the objective and eyepiece, its effect 

 is to reduce' the magnifying power of any objective about one-half; 

 at the same time shortening the "working distance" of the objective 

 materially. Used with a 2-inch, 3-inch, or 4-inch objective it practi- 

 cally makes them 4-inch, 6-inch, or 8-inch, respectively. Its effect is 

 not good with high powers, where of course it is not needed. 



By using a similar lens as an objective, in connection with the 

 reducing lens, I have a very low power of only five or six diameters 

 and three and a half inches working distance. This power takes into 

 the field of the lowest eyepiece an object, eighty-eight hundredths of 

 an inch in diameter, with a depth of focus of nearly half an inch. 



This lowest power I have found very useful in viewing whole flowers, 

 large sections. Zoophytes in glass troughs, etc., as all their parts, both 

 breadth and depth, were brought into view at once. 1 do not bring 

 this forward as an " optically " perfect instrument, but as a conven- 

 ient method of producing very low powers for certain purposes. 

 Below I give a table of diameters with the different eyepieces. 





Lowest 

 power. 



B4nc7i 

 objective. 



Eyepiece A. . 



. . 5 diameters. 



10 diameters. 



" B. . 



. . 10 " 



18 " 



" C. . 



. . 18 " 



32 " 



It will be seen that the lowest power gives only the magnifying 

 power of the eyepieces used, and the 3-inch only about double the 

 power. 



