82 Transactions of the Royal Microscopical Society. 
II.—Observations on Professor Abbe’s Experiments illustrating 
his Theory of Microscopic Vision. 
By J. W. Srepuenson, F.R.A.S., Treasurer R.M.S. 
(Read before the Rovat Microscoricay Sociery, January 3, 1877.) 
Pruare CLXXIII. 
In my opinion, ‘the very important theory of microscopic vision 
which has been enunciated by Professor Abbe,* has not received, 
in this country, the attention it pre-eminently deserves. The 
theory to which I refer, is, that the microscopic images produced by 
certain objects of minute detail, such as diatoms, scales of insects, 
EXPLANATION OF PLATE CLXXIII. 
Fic. 1.—Fine grating used as the object on the stage of the microscope. 
Fic. 2—A ppearance presented on removing eye-piece and looking down tube, 
showing central and spectral images. 
Fic. 83.—Diaphragm with tliree slits in position, shutting out certain spectral 
images, and making those produced by coarse and fine lines identical. 
Fic. 4.— Appearance presented on examination of object (Fig. 1) when ex- 
amined under the latter condition ; fine lines in normal condition, coarser doubled 
in number. 
Fic. 5.—Diaphragm excluding all the spectral rays from finer lines, and all 
except the two adjacent to central beam from the coarser lines. 
Fic. 6.—Shows the coarse striz alone; the finer invisible in consequence of 
all their spectra being excluded. 
Fie. 7.—Diaphragm excluding all spectral rays. 
Fig. 8.—No lines visible in consequence. 
Fic. 9—Diaphragm excluding all the spectra of Fig. 2, except the fourth of 
the coarse and second of fine grating. 
Fic. 10.—Appearance presented; coarse lines quadrupled and fine lines 
doubled in number. 
Fic. 11.—Effect produced by light of extreme obliquity on parallel lines of 
such fineness as to have nearly reached the limit of resolvability; the illuminating 
pencil at the edge of field, and only the more refrangible rays of spectral image 
remaining in the field at the opposite side. 
Fic. 12.—Appearance presented in tube by single valve of P. angulatum ; 
light central. 
Fic. 13.—Fine crossed grating (60°) used as object on stage of microscope. 
Fic. 14.—Spectral aspect produced by crossed grating; the spectra within 
smaller circle identical in form with P. angulatum, Fig. 12. 
Fic. 15.—Diaphragm in position admitting central beam and three spectral 
rays; the imaginary lines joining them crossing each other at right angles. 
Fic. 16.—Appearance of Fig. 13 under these conditions; the lines crossing 
each other at right angles at distances inversely to those of the spectra (,/ 3 ¢ 1). 
Fic. 17.—Square grating. 
Fie. 18.—Appearance presented in tube. 
Fic. 19.—Diaphragm admitting central and one spectral image. 
Fic. 20.—Consequent disappearance of all real lines, and substitution of 
diagonal lines at right angles to admitted rays. 
* A valuable translation, by Dr. H. E. Fripp, of Professor Abbe’s “ Contribu- 
tion to the Theory of the Microscope, and the Nature of Microscopie Vision,” 
appeared in the ‘ Proceedings of the Bristol Naturalists’ Society,’ new series, vol. i. 
part 2. 
