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be formed. Reflection and refraction, therefore, nnder such 
conditions, are unimpaired by the diffraction which would 
ensue, if the interspaces of the object were wider. Hence, 
scratches, &c., on lenses and reflectors, when separated by 
less than a wave length, produce no diffraction spectra, and 
therefore allow of refraction or reflection taking place with- 
out impairment. The process of polishing has for its object, 
merely the substitution of fine scratches of non- diffracting 
dimensions for inequalities of a grosser character. 
If now we place on the stage of the microscope any 
object characterised by a structure of fine lines, dots, &;c., 
illuminate it by transmitted light (using a very small 
aperture of the diaphragm), and, removing the eyepiece, 
look down the tube of the instrument, 
we see, not only a central colourless 
image of the small aperture in the 
diaphragm, but also on each side of this 
a series of spectral images of the same 
object. (1) These spectral images are 
situated near to the central image and 
to each other, if the diffracting structures of the object 
is coarse, their separation increasing directly as the lines 
or dots approximate. If, therefore, an object is viewed 
presenting two series of lines, the one double the distance 
apart of the other (2), we perceive on looking down the 
tube a double series of images, one consisting of images 
twice the distance apart of the other (3). And, on replacing 
the eyepiece, a magnified image of the two series of lines 
