ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
241 
taneous rotation of the two nicols has been adopted, has already been 
described in this Journal, 1892, p. 668. In its latest most perfect form 
it is represented in fig. 43. This model is provided with a rotating 
stage and centering arrangement for the objective. 
In another model, having the same form and size, the stage is fixed. 
Fuess also supplies a cheap instrument, non-inclinable, and without fine- 
adjustment, but having the same arrangement for the simultaneous 
rotation of the nicols. 
(2) Eye-pieces and Objectives. 
A Modern Microscopic Objective.* — Mr. H. Orford looks upon the 
Abbe diffraction theory as one of the “ untenable theories ” which were 
advanced “ years back, when the instrument was known only to a few 
scientists.” He is willing to admit, however, one good effect which has 
resulted from it, viz. that it has settled for ever striving after useless 
magnifying power in objectives with small apertures, and has led 
opticians to construct lower powers of large aperture. He endeavours 
to demonstrate the benefit derived from the larger aperture by reference 
to the pinhole experiment of Lord Kayleigh, which consists in looking 
at a piece of fine wire gauze through a small and a large pinhole in a 
black card. If the gauze be gradually removed until the meshes can 
be no longer seen through the small pinhole, they will again become 
visible on looking through the large pinhole. That this result is not 
merely due to the greater intensity of the light is shown by another 
experiment, which consists in looking at the gauze through two scratches 
(one vertical and the other horizontal), about 1/16 in. long, on a piece 
of blackened glass. The gauze is held so that the wires are horizontal 
and vertical to the scratches on the glass, when at a certain distance it 
will be found that the vertical scratch will only show the horizontal 
wires, and the horizontal scratch only the vertical wires. 
The result of the author’s experience as a manufacturer of lenses of 
some years’ standing is that a Microscope objective should have large 
aperture, but that aperture is worse than useless unless it is properly 
corrected. 
Abbe’s Spectroscopic Ey e-piece, j — This instrument (figs. 44 and 45) 
is specially constructed for the examination of absorption-spectra of 
microscopic objects. The achromatic upper lens is adjustable so as to 
focus on the slit between the lenses. The mechanism is actuated by 
a screw, so as to effect the symmetrical contraction and expansion 
of the slit. The screw H serves to limit the length of the slit, so that 
the image of the object may completely fill it when the comparison- 
prism is inserted. 
Above the eye-piece is placed an Amici prism of great depression 
which may be turned aside about the pivot Jc , so as to allow of the 
adjustment of the object being controlled, the prism being retained in 
its axial position by the spring catch L. A scale of wave-lengths is 
projected on the spectrum by means of a scale-tube and mirror attached 
* Journ. New York Micr. Soc., xi. (1895) pp. 106-10. 
f Zeiss’ Catalogue, No. 30, 1895, pp. 88-9. 
