210 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



viz. the power of optically separating two structures superimposed upon 

 one another. In the words of Dr. Abbe, a wide aperture then becomes 

 ^n optical microtome. 



Power of a Microscope.* — The Enghsh Mechanic quotes the 

 following simple method of finding the magnifying power of a lens, as 

 given by 0. W. Nieman in the Scientific American. Take a good 

 watch and measure the exact length of the hour-hand, that is, from 

 the tip to the centre of the pivot. Suppose this is found to be 0* 57 in 

 Then the path of the point in twelve hours will be 2 tt ' 57 = 3 • 58 in. 

 Therefore the point will move y^-Q in every 2 min. 1 sec. To the side 

 of the barrel of the Microscope fasten a piece of sheet metal, with a 

 small hole in one end, and bent as shown (fig. 20). The distance 

 between the hole and the centre of the eye-piece must be the same as 

 the distance between the experimenter's eyes. On the table below the 

 hole, fasten a ruler exactly 10 in. below the line joining hole and centre 



G 



^ 





Fig. -20. 



of eye-piece. The Microscope is now focused down on to the watch- 

 face, tJie crystal cover having been removed. If the experimenter 

 looks in the Microscope with one eye and through the hole with the 

 other, the watch-hand and the ruler will appear superposed. Note the 

 exact point on the ruler where the hand is at any instant. Then let 

 2 min 1 sec. pass, and again note the positions. If the distance moved 

 is 2" 5 in. it will mean a magnification of 2*5 in. -f- 0*01 in. = 250 

 diameter. A scale may now be constructed on a slip of cardboard, in 

 which every | in. will represent njoo' ^^' i^^ the object. The scale 

 would have to be brought into the field of view and superposed on the 

 object in the same way as the ruler was used. 



Dark-ground Illumination.t — A. E. Conrady, as the results of 

 theory and experiment, enumerates the following principles concerning 

 dark-ground resolution. 1. In order to obtain the utmost resolving 



* English Mechanic, xcvi. (1913) p. 564 (1 fig.). 



t Joiirn. Quekett Micr. Club, xi. (1912) pp.. 475-80 (2 figs.). 



