100 THE AMERICAN MONTHLY [June 



source of light. They interfere and form the image seen 

 by the eye-piece, in the Fresnel and Abbe manner. Re- 

 moving the stage grating, and replacing the coarse one 

 over the flame, its focal image is now the object. Owing 

 to the heterogeneity of the rays, this serial image emits 

 no spectra — there neither are nor can be any such. But 

 it is perfectly resolved. Here we have a resolution of 

 3,000 or 6,000 lines per inch that has no place at all in 

 the •* spectrum" theory; which therefore can be no 

 complete theory of microscopic vision, though it has an 

 important place in it. 



Using reduced photographs of perforated zinc, I have 

 similarly used their aerial images in comparison with 

 P. angulatum on the stage. Only approximately in one 

 respect, because the difficulty of getting suflBciently 

 reduced photographs prevented use of the same illumina- 

 ting cone in the two cases. But there is no doubt about 

 the results in all important respects. 



As another expedient, we may place beneath the slide 

 a sheet of finely-ground glass. This ground surface 

 refracts and reflects the light in countless phases and 

 directions through the object, the waves issuing there- 

 from with similar heterogeneity of character. Here also 

 we must have at least a very considerable degree of 

 approximation to the nature of self-luminosity ; nor can 

 we get from such illumination any of the well-marked 

 " spectra " or out-of-focus interference-fringes, familiar 

 to us with the Abbe method. The difference in char- 

 acter of illumination by such methods, is so great, that if 

 the "spectrum" theory be completely true, there should 

 at least be a uniform and vast deterioration in the image 

 of an object thus illuminated. On the contrary, with all 

 good lenses of moderate aperture, and slides with any 

 fair amount of opacity in details, such an image is about 

 the loery best we can get. The excellence of this method 

 of illuminating was first shown many years ago. By its 



