﻿74 Mr. W. B. Croft : Some 



Diffraction in a Microscope. 



Some notice of this important part of the subject is neces- 

 sary, although it may not be supported by much illustration 

 or experiment. 



It is well, first of all, to be clear as to compound pencils of 

 light which are more or less on the same axes. In a straight 

 line lie A, B, C, U, E, a candle, a device in wire, a convex 

 lens, the images of the candle and the wire : here are two 

 pencils much intermingled : A is in focus at D, but the light 

 here is also impregnated with the shadows of B, which are 

 indefinite and unfocussed, covering a larger area than when 

 focussed atE ; and at this point there is also the indefinite 

 unfocussed light of A. 



Very often the object on the stage of a microscope diffuses 

 the illumination and becomes a new source of light, and there 

 is no need to consider pencils from tbe original source of 

 light. But here we will take the case of a microscope with a 

 diffraction- grating on the stage and a small opening in the 

 diaphragm below. Light passes on to the objective from the 

 grating as if from several small apertures ranged side by side, 

 in pairs of corresponding spectra on either side of the central 

 point, which is white. The objective makes images of these 

 just above itself, and one image of the grating farther on at 

 the top of the tube. If the eyepiece is removed, the eye is 

 able to focus the rays diverging from these images and to see 

 them clearly. But the eyepiece when in its place could not 

 focus such diverging pencils : their light spreads out and fills 

 the field of the eyepiece with general illumination. The visi- 

 bility of the detail of a finely divided object depends upon 

 shadows thrown by oblique rays. In the case of a grating 

 these oblique rays are so definitely arranged that they go 

 either to the first, second, or higher spectra, and to nothing 

 between. The central pencil comes through the grating nor- 

 mally in parallel rays, and carries with it no appreciable im- 

 pression of the detail ; experiment shows that on placing a stop 

 above the object-glass to cut off the spectral images, the lines 

 of the grating can no longer be seen, although there is still 

 good illumination. Now the first lot of oblique rays, while on 

 their way from the grating to its focus in the eyepiece, take 

 their course through the first spectra ; but they are un- 

 focussed as regards the grating at that position just above 

 the objective, covering a larger area than the spectra. In 

 theory perhaps it is conceivable that by stopping out the 

 spectra alone, and not this larger area, there might be a 

 balance of rays which would betray the shadows at the eye- 



