388 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
m fig. 41 and seen also in fig. 40), causing them to move exactly together 
outwards, or in the reverse direction. 
A great advantage of the apparatus is that instead of being made of 
brass, and therefore very heavy, the eye-piece 
tubes are of aluminium, and the prism-box of 
ebonite ; an admirably light eye-piece is thus 
obtained, which in that respect leaves nothing to 
be desired. 
Hartnach also made some binocular eye-pieces, 
but we have not been able to obtain the materials 
for their description. Dr. Dippel gives * the 
following description of two of them, but his woodcut of the first, 
which he describes as having four Kiddell prisms, is obviously not 
correct, as it figures an eye-piece like that of Verick, which will not take 
the Eiddell prisms. 
“ In the older Hartnack binocular, which is inserted into the Micro- 
scope-tube by means of an adapter, the duplication of the image and 
halving of the pencil of rays takes place in the course of the pencil 
between the objective and the position of the real objective image. 
Eiddell’s arrangement of four prisms serves for this purpose, the two 
eye-pieces being rigidly connected with the two prisms, which direct 
upward the twice totally reflected rays parallel to the axis of the 
Microscope. The adjustment of the eye-pieces to suit the eyes of the 
observer is effected by mechanism put in motion by a screw-head. More 
recently, Dr. Hartnack has constructed a somewhat more complex 
binocular eye-piece, which gives splendid images with a small field of 
view, and, as far as I could ascertain, ^ agrees in principle with the Tolies 
apparatus, inasmuch as a prism, over a lens-system in the lower tube 
acting as eye-piece, divides the image into two erect images, which are 
observed through two ordinary eye-pieces converging below and movable 
by rack and pinion in the direction of their long axes.” 
Fig. 41. 
(3) IlluminatiniT and other Apparatus. 
Screw Eye-piece Micrometers. — Much discussion has taken place 
in recent years on the subject of the relative accuracy of the different 
eye-piece micrometers, that is, between the fixed glass-plate micrometers 
on the one side, and the movable screw or spider’s- web (filar) micro- 
meters on the other. 
In all' these discussions the only screw-micrometer that has been 
referred to is the ordinary English form, with two spider lines, in which 
the optical part of the eye-piece is fixed immovably in the optic axis, 
while one of the spider lines traverses the field of view by the action 
of the screw. There are, however, some refinements of this apparatus 
to which attention may be called. 
An important defect of the ordinary form of screw-micrometer arises 
from the fact that the measurements are not effected with the centre of 
the eye-piece alone, but use is made of the excentric parts of the lenses. 
The result of this is that the image of the object is subjected to more 
or less distortion, as its various parts are magnified differently, accordiu g 
* L. c,, pp. 598-1). 
