160 THE MICROSCOPE AJSTD ITS REVELATIONS. 



If the square grating be focussed, and its image examined by looking into the 

 tube of the Microscope without the eye-piece, the diffraction-spectra will exhibit 

 the regular arrangement shown at R; the round image being in the centre of the 

 field, and the ovals being disposed in five rows at equal distances. 



Experiment 5. A diaphragm being interposed (L) with an oblique slit just 

 large enough to admit the central circle and one of the diffraction spectra, and 

 the eye-piece being replaced, the real rectangular lines will not be seen at all, but 

 the field will be traversed by oblique spectral lines (o), whose direction is at right 

 angles to that of the slit. 



If the image of the rhombic grating be examined in the same manner as that 

 of the square, it will exhibit the arrangement shown at P (the dotted inner circle 

 being here disregarded). 



Experiment 6. By using a diaphragm with a single slit in the direction of one 

 of the diagonals of the rhomb, a Microscopic image will be presented from which 

 both sets of real lines are entirely absent, whilst a single set of spectral lines is 

 seen, whose direction is at right angles to the slit, that is, in the direction of the 

 other diagonal. 



Experiment 7. Again, by using a diaphragm with two slits at right angles to 

 ach other (Q), the Microscopic image will show two sets of spurious lines cross- 

 ing one another at right angles (M), in the directions of the two diameters of the 

 rhomb, the real lines being altogether invisible. 



Experiment 8. A very singular effect is produced by the use of a single circu- 

 lar diaphragm, whose aperture is reduced so as only to include the six spectral 

 ovals lying within the dotted circle at P; for on then replacing the eye-piece, the 

 .entire field is seen to be marked out in hexagons. 



Now if a valve of Pleurosigma angulatum be focussed with central illumina- 

 tion under an Objective of sufficiently high power and large aperture, and the 

 eye-piece be removed, there will be seen on looking down the body a bright cen- 

 tral beam, with six colored spectra arranged round it just as in the interior 

 spectrum (P) of the rhomboidal grating; the reason that no other spectra are seen, 

 being that the approximation of the markings carries these six spectra to the 

 extreme border of the field of even the largest-angled Objective. An Objective 

 of smaller angle will not show them at all with central light; but if oblique light 

 be used, the circular beam is carried to one margin, and a single spectral oval is 

 seen at the other; and the recombination of these suffices to make one set of lines 

 visible. Again, by recombining, by means of appropriate diaphragms, any three 

 of the spectral ovals, or any two of these with the central beam, the very same 

 part of the valve may be made to show a great variety of appearances such as 

 are actually seen in different parts of the same valve under the same illumination 

 (Fig. 166). 1 



The foregoing experiments, then, entirely confirm the general con- 

 clusions drawn from those of the previous series, (1) as to the entire 

 distinctness in character between the images Dioptrically formed of the 

 general outlines and larger details of Microscopic objects, and the 

 representations of their finer details which result from the reunion of 

 their Interference-spectra; and (2) as to the very limited trustworthiness 

 of the latter, when the minuteness of the structure occasions such a wide 

 separation of the ' diffraction-spectra/ as limits the number thus com- 

 .bined. Thus it becomes clear (1) that the 'resolving power' by which 

 closely-approximated lines or other markings are separated, increases 

 .(the completeness of the corrections for Spherical and Chromatic aberra- 

 tion being presupposed) with the Angular Aperture* of the Objective; 



1 See the original Memoir by Prof. Abbe, ' Beitrage zur Theorie des Micro- 

 scopes,' in " Archiv fur Microscop. Anatomic," Bd. ix. (1874), p. 418; Dr. H. E. 

 Fripp's translation of it in the "Proceedings of the Bristol Naturalists' Society," 

 N.S., Vol. i., part 2(1875>; extracts from Dr. F.'s translation in " Monthly Microsc. 

 Journ.," Vol. xiv. (1875), pp. 191, 245; also Mr. Stephenson's 'Observations' 

 thereon to which the Author has been specially indebted op. cit., Vol. xvii. 

 {1878), p. 82; and Mr. F. Crisp " On the Influence of Diffraction in Microscopic 

 Vision," in "Journ. of Quekett Club," Vol. v., p. 79. 



2 The term Angular Aperture is to be understood as differentiated from " Angle 

 of Aperture" ( 10), by the allowance made for the modification in the course of 



