332 THE NORTHERN MICROSCOPIST. 

be seen. If only the central rays pass to the eye, and the diffracted 
rays be excluded, no minute structure will be visible. The central 
light and at least one diffracted ray, or two diffracted rays only, 
are essential to the visibility of any structural detail ; and the like- 
ness of the image to the real character of the object will depend 
upon the number and position of the diffracted rays that are com- 
bined in the visual image. Diffraction spectra can be seen in the 
microscope, and have been exhibited, together with their resultant 
images, to the R. M. Society, the Quekett Microscopical Club, and 
elsewhere. : 
If a plate of glass with fine ruled lines, such as are represented 
in Fig 40 in the magnified image, be placed upon the stage of the 
Fig. 40. Fig. 41. microscope, and a beam of 
white light from the mirror 
made to pass through a small 
opening in a diaphragm be- 
tween the mirror and the 
stage; upon removing the 
eye-piece and looking down 
the tube, spectral images of 
the source of light will be 
seen on each side of the 
central beam, as in Fig. 41; 
the closer images of the up- 
per line being formed by the 
wider lines of the plate, and 
Fig 42 the more distant images by 
the closer lines. Ifa diaphragm be now placed at the back of the 
objective, covering all the spectra, and allowing only the central 
rays to pass, upon replacing the eye-piece the lines on the plate 
will have disappeared. If we now place another diaphragm at the 
back of the objective, so as to exclude all but the central rays and 
the outermost spectra, upon replacing the eye-piece we shall see 
more lines than are in the object, the appearance being as in Fig. 
42. ‘The false lines are the result of the phenomenon of inter- 
ference or intermixture of the luminous waves. On this principle 
a great variety of effects may be produced from object-plates ruled 
to different patterns and an adequate manipulation of the spectra 
produced by them. Upon the admission or exclusion, more or 
less, of the diffraction rays, as they are bent off in angular groups, 
depends the capability of the microscope to show things as they 
are in nature, and therefore affects, not only the resolving power, 
but the delineating power of the instrument. 
Professor Abbe says that very minute structural details, as a rule, 
“cannot be interpreted as morphological but only as physical cha- 
racters, not as images of material forms, but as signs of material 


