200 THE AMERICAN MONTHLY [Nov 



irregularly opaque structure between, the "lattice work" 

 openings in the Pleurosigma angulatum. This is at 

 least 300,000 diameters beyond that ever before achieved 

 but I think this is but the beginning of the new field 

 into which I have entered. A practical test exhibited to 

 practical men led to the investment of sufficient capital 

 to put this instrument upon a commercial basis, and such 

 instruments will be upon the market as soon as 'all the 

 details of manufacture can be arranged and patents ob- 

 tained. The most important improvement beyond the 

 ones just mentioned is that of making a microscope 

 specially adapted to mono-chromatic light. , This obviates 

 the necessity of using lenses for chromatic aberration. 

 The advantage is obvious. It makes not merely a 

 cheaper microscope but it makes one in which the defini- 

 tion is improved. By using a large area of mono-chro- 

 matic light made by large quartz lenses, and then focuss- 

 ing this to a narrower beam rendered parallel by suita- 

 ble lenses, I expect to get into the microscope from one 

 hundred to one thousand times the amount of light that 

 has ever been put into it before. If any one will take a 

 Bausch and Lomb microscope, with a one inch ocular, 

 and put in it a one-sixth inch Bausch and Lomb objective 

 and focus it upon a pleurosigma, using a ray of con- 

 densed sunlight, because ordinary sunlight would not be 

 intense enough, or using a powerful arc-lamp, with a 

 sixth-inch condensing lens and bringing the beam down 

 to one-quarter of an inch in diameter, and then parallel- 

 izing the rays, he will have the first part of the experi- 

 ment complete. The definition will not be so good with 

 an arc-lamp as with sunlight because it is impossible to 

 have the arc-lamp rays entirely parallel. This owing 

 to the fact that the light from an arc-lamp does not come 

 from an absolute point, but from an area. If the experi- 

 menter will then remove the front lens of the ocular and 

 adjust upon the focal plane of the image in the ocular a 



