198 THE AMERICAN MONTHLY [Nov 



Monochromatic light makes possible another important 

 improvement. I do not mean the use of the well-known 

 sub-stage spectroscopic attachment, but of large prisms 

 in series so as to give a large area of actinic rays of one 

 color, and then the focussing of these rays to a diameter 

 several hundred or thousand times less, than making- 

 them parallel, and sending them into the substage con- 

 denser as you would a beam of sunlight from the helio- 

 stat. Such rays obviate the necessity for the usual addi- 

 tional lenses for correcting chromatic aberation. As is 

 well known, the different colored rays come to a focus at 

 different distances from the lens. In microscopes 

 of the usual pattern it requires several lenses, in 

 addition to the ones required for resolution and 

 ' magnification, to correct this and make the rays come to 

 one focus. If rays of one color are used only, then no 

 corrections need be made for the rays of the colors not 

 used. This requires less thickness of glass for the rays 

 to pass through, and consequently the light will be 

 stronger, and this again slightly extends the formerly 

 assigned limits. As far as I know the microscope which 

 I am now having built is the first one that has been 

 made especially for one-colored light. It requires cor- 

 rection for only one color, and for spherical aberration, 

 and this latter presents less difficulty for larger than 

 for smaller lenses. 



The first statement generally made by microscopists in 

 discussing the new double microscope is one which af- 

 fects profound pity for the man who could presume to do 

 what has not been done before. This is generally coupled 

 with the statement that every microscopist well knows, 

 and has known for a long time, and that every beginner 

 in microscopy ought to know, that if you produce more 

 magnification than has hitherto been produced that you 

 cannot see the image ; and that, therefore, my claims to a 

 higher magnification are erroneous. Now the very gist 



