54 JOURNAL OF THE [March, 



better definition ; and many new forms were seen, and many 

 other forms were better seen than ever before. 



Yet a still further advance was made, when a medium homo- 

 geneous with the glass, was used, or of the same refractive and 

 dispersive index as glass, instead of water ; and the lenses 

 that give the best definition and show minute objects the clearest 

 are those called "homogeneous-immersion objectives." These 

 lenses, however, with the highest powers in use, are not at all 

 suited to a public exhibition — every motion being magnified as 

 well as the object ; and the delicate adjustments necessary to 

 keep this object in focus, preclude the use of such lenses, except 

 in a very quiet place. 



It is often asked, " What are the limits of the powers of a 

 microscope ? Have they been reached by opticians ? " From our 

 present standpoint, with the known difficulty of grinding small 

 lenses, also with the difficulty of using them satisfactorily, it 

 would not seem possible to go much further in this direction. 

 But we know too little of the forces in this universe, usable by 

 man, to form a correct judgment. 



The delicacy of the human ear was not conceived of, until the 

 telephone demonstrated that the ear can hear and appreciate 

 vibrations inappreciable by the most delicate instrument ; and 

 that acoustic vibrations can be transformed into electrical, and 

 back again, until there is, virtually, no limit to the use of the ear 

 but the imperfection of insulation. 



In a letter published a few days since, Mr. Thomas A. Edison 

 states that there could be no difficulty in talking over a wire 

 stretched to the moon, and that over the treeless and dry plains 

 of the West, it is as easy to hear one thousand miles as a few 

 miles in a crowded city. 



In the same way, the delicacy of the human eye is not yet 

 appreciated. Prof. Rood, some years since, by a very ingenious 

 mechanical method, — described in the " American Journal of 

 Science," — demonstrated that the eye is capable of seeing with a 

 flash of light lasting the fifty-millionth part of a second. 



We have, so far, only used the refractive powers of translucent 

 substances — as glass — to see minute things ; but the future may 

 give us, by other means, a method of seeing — as we now have a 

 method of hearing — infinitely preferable to the one in present 

 use. 



