156 THE MICROSCOPE AND ITS REVELATIONS. 



of the opposite sides uniting at the broad end of the scale, where they 

 generally project as bristle-like appendages beyond the intermediate mem- 

 brane; and they are crossed transversely by fine markings, which are 

 probably ridge-like corrugations of their membrane, these also existing 

 on both surfaces of the scale. The attention of Dr. Woodward having 

 been drawn by Dr. Anthony to the presence in these scales of three uni- 

 form parallel rows of beads in every interspace between two adjoining ribs> 

 he was at first inclined to believe that the markings are real, represent- 

 ing an actual structure in the scale; but having obtained an excellent 

 Photograph of it by monochromatic sun-light, under a power of 1,350 

 diameters, he was led to alter his opinion, and to regard them as pro- 

 duced by the crossing of the transverse markings by longitudinal diffrac- 

 tion-lines, conditioned by the longitudinal ribs and parallel to them. 1 

 His chief reasons for so regarding them were (1), that "the longitudinal 

 diffraction-lines are clearly seen alike in the Microscopic image and in 

 the Photographs, to extend into empty space beyond the contour of the 

 scales, almost as far as the ends of the bristles in which the parallel 

 ribs terminate;" and (2), "that they vary in number with varying ob- 

 liquity of illumination, so that in the same scale two, three, four, and 

 five rows of beads can be seen, and photographed at pleasure, in every 

 intercostal space." The true appearance, Dr. Woodward considers, is 

 given when the Achromatic Condenser is so adjusted that its light is- 

 either central or slightly oblique in the longitudinal direction of the 

 scale. 



157. The recent researches of Prof. Abbe of Jena appear to have con- 

 clusively proved that Diffraction has a most important share, previously 

 altogether unsuspected, in the formation of the Microscopic images of 

 very closely approximated lines or other markings, in objects viewed 

 under high magnifying powers of large Angular aperture. All that has- 

 been hitherto said of the formation of Microscopic images, relates to- 

 such as are produced, in accordance with the laws of refraction, by the- 

 alteration in their course which the Light-rays undergo in their passage 

 through the lenses interposed between the object and the eye. These- 

 dioptric images, when formed by lenses free from Spherical and Chro- 

 matic aberration, are geometrically correct pictures, truly representing ther 

 appearances which the objects themselves would present, were they en- 

 larged to the same scale, and viewed under similar illumination. And 

 we are fully justified, therefore, in drawing from such Microscopic images 

 (provided that they are free from diffraction-spectra) the same conclu- 

 sions in regard to the structure of the objects they picture, as we should 

 draw from the direct vision of actual objects having the same dimensions. 

 There is, however, an optical limit as to the completeness of such images 

 in regard to minute detail; as it appears from the theoretical researches 

 of Profrs. Helmholtz and Abbe, that no amount of magnifying power 

 can separate dioptrically two lines, apertures, or markings of any kind, 

 not more than l-2500th of an inch apart. The visual separation or ' reso- 

 lution ' of more closely approximated lines or other markings is entirely 

 the result of diffraction; the Objective receiving and transmitting, not 

 only the ordinary dioptric rays, but the ' inflected ' rays whose course has 

 been altered in their course through the object by some peculiarity in the; 

 disposition of its particles. These rays, when acted-on by the Objective,, 

 produce ' diffraction-spectra;' the number and relative position of which 



1 " Monthly Microsc. Journ.," Vol. xv. (1876), p. 253. 



