94 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Monochromatic Light.* — Dr. Wilibald A. Nagel indicates the 
various difficulties concerned in the employment of prisms for the 
obtaining of monochromatic light, and recommends instead the use of 
Str alilen- filtern” Of these he thinks fluids have many advantages 
over coloured glass or gelatin films, lie gives a complete list for all 
parts of the spectrum of fluids which can be used for this purpose. 
(4) Photomicrography. 
Neuhass’ Lehrbuch der Mikrophotographie.j — Dr. Neuhass has 
brought out a second edition of this book, in which he tries to bring the 
subject up to date. The work is divided into eight sections, viz. (1) The 
microphotographic apparatus. (2) Objectives and oculars. (3) The 
light source. (4) The illumination. (5) Arrangements for special pur- 
poses. (6) The negative image. (7) The positive image. (8) Prepara- 
tions ; significance of microphotography ; micropliotograms. This last 
subsection comprises a review of the chief microphotograms taken during 
the last ten years by operators of various nationalities. Several of the 
sections contain a historical subsection. 
(6) Miscellaneous. 
Theory of the Microscope.^; — Herr Karl Strehl grounds his paper 
on this subject upon the formulae obtained by him in his ‘ Theory of 
the Telescope,’ in conjunction with the proposition (Abbe, Dippel, 
Czapski, Strehl) that the Microscope = lens -f- telescope. 
As a preliminary he refers to the papers of Lord Rayleigh' § and 
Lewis Wright. |j He criticises Wright’s view that “for irregular and 
unperiodic detail wide-cone illumination is the most useful, because it 
brings the object into a condition approximating to theoretical self- 
luminosity.” He only admits this statement when an image of a self- 
luminous surface can be formed in the object-plane by means of an 
apochromatic condenser, whose N.A. is as much as possible greater than 
that of the apochromatic observation system. For the limit of fineness 
of resolvable detail is accidentally determined for self-luminous objects 
in the same way as for those objects which are illuminated with ex- 
tremely oblique plane waves, and indeed depends only on the wave- 
length and the numerical aperture. For selfduminous objects he finds 
the limit given by £ = (2 • 95 : 2 -n) . (A. : n sin a), and for extremely 
oblique light Abbe’s theory gives £ = A. : 2 n sin a ; both values are equal 
up to G degrees. Strehl’ s previous value of £ = (3* 20 : 2 7r) . (A. : n sin a) 
gives an experience limit of 3 to 4 degrees for the observer’s eye. Ho 
admits that with this mode of illumination images may be less distinct, 
but that they are more truthful. He has found wide light-cones give a 
notable resolution of the finest details of butterfly scales, and agrees 
that not only in the case of coloured isolated structure-elements is such 
illumination advantageous. He points out that his statement, “the 
influence of sj lieiucal aberration has been considerably over-rated in 
objectives,” was misunderstood by Wright, as he was only referring to 
telescope objectives. 
* Biol. Centralbl., xviii. (189S) pp. 619-55 (with spectral maps), 
t Bruhn, Braunschweig, 1S98, 266 pp., 62 woodcuts, 2 autotypes, and 2 pis. 
X Zeitschr. 1. Instrumenteuk , 1898, pp. 301-17. § Phil. Mag., xlii. (1896) p. 167, 
|| Op. cit., xlv. (1898) p. 480. Of. this Journal, 1896, p. 681 ; 1898, p. 592. 
