ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
679 
ment, and is fitted with a movable substago condenser and low-power 
objective. The adjusting substage is so made that a spot-lens or dark- 
ground illuminator can be used, also a Lieberkuhn for illuminating 
opaque objects. A large Nicol cau bo used for showing crystals ; rock 
sections by means of polarised light show well on the screen. The ease 
with which the Microscope can be shifted to the lantern front for showing 
photomicrographs makes the instrument especially useful to the lecturer. 
The third front is for spectroscopic work, and is fitted with slit, 
direct-vision prism, and achromatic focussing lens, which project a 
brilliant spectrum on the screen without the necessity of placing the 
lantern at an angle. A comparison prism is also fitted, by which at 
the same time an image is projected on the screen of any medium of 
which the absorption spectrum is being shown. 
Fig. 118. 
The whole of this part of the apparatus can be lifted off, leaving a 
clear base-board, so that a vertical attachment, or photochromoscope, 
elbow polariscope, or any front can be substituted that may be wanted 
directly. The condenser is mouuted on a hinge, aud can be removed or 
put into position instantly. Either lime-light or the electric arc can be 
used in this lantern ; and as it is made throughout in Messrs. Newton 
& Co.’s workshop, it is perhaps unnecessary to say that every detail has 
been carefully arranged, and that the workmanship is of a high class. 
(6) Miscellaneous. 
Abbe’s Theory of the Microscope.* — Karl Strehl, of Erlangen, pro- 
pounds the seven following propositions, which seem to him to be of 
fundamental importance for rightly understanding Abbe’s theory of the 
Microscope. 
“ (1) Of the continuations of the plane-wave which falls perpen- 
dicularly on the primary optic axis and illuminates the object, only the 
direct continuation is a plane-wave, the deflected ones are by their very 
nature not plane- waves. 
* Central-Ztg. f. Opt. u. Mech., No. 18, pp. 71-2 (2 figs.). 
