10 THE AMERICAN MONTHLY [Jan 



methods,! may leave to the reader's own observatiou how 

 it acts here. 



It is curious to observe that in this method we have em- 

 ployed a comparatively very narrow cone of light from the 

 condenser, with an objective of large aperture, just the 

 exact reverse to what we did in our first method. 



That in this method the color of the background is sim- 

 ply determined by the color of the only light wiiich gets 

 into the microscope-objective, when there is no object 

 placed in the path of the light rays (to wit, blue in our 

 example), stands to reason (fig. 7). 



It only remains for us now to see where the use of mul- 

 tiple color illumination comes in, and what is its scope. 

 For the pretty results thereby obtainable, though* very 

 good in their way, and calculated to call forth expressions 

 of delighted surprise from our non microscopic friends, 

 are not the most worthy object of the microscopist's am- 

 bition. To see a ruby-red rotifer disporting itself in a 

 deep green sea, to look at muscle fibres with alternate red 

 and blue bands, in short, to see our objects highly color- 

 ed, like the Lord Mayor's Show, what is the use ? 



The use may be summed up by saying that we increase 

 cur knowledge of the object by, increased ability to see 

 it and by increased ability to draAv conclusions from that 

 which we see. • When we go out on a sunny day in the 

 country, we put on a broad-brimmed hat, so that the light 

 of the sun may be kept from our eyes, and we can see the 

 landscape better. If we want to see particularly well we 

 even shade our eyes further by holding our hands up to 

 the brim. That is just what we usually do not do when 

 we look through tiie microscope, for we gaze at the full 

 glare of our light, and if it is too strong we merely shut 

 • ome of it off without stopping to distinguish between, 

 or to consider whether, it is image-forming or background 

 forming light; nevertheless we expect our eyes to dis- 

 tinguish all they might be capable of doing. 



