15 
^Compensating Eye-pieees. 
All objectives of considerable aperture, from their peculiar construction 
(hemispherical fronts) exhibit certain colour defects in the extra -axial portion of 
the visual field (chromatic difference of the magnification), even if perfectly achro- 
matic in the centre. The differently coloured images which combine to 
form the final image (Dippel, 2'"^ ed. p. 225) are of different dimensions, the 
blue image being greater than the red one. Whether an image be directly 
projected by such an objective or whether it be examined with an eye-piece (even 
of the achromatic or so-called aplanatic type), colour fringes will be observed, 
increasing towards the margin of the field. 
This peculiarity is also possessed by -the apochromatic objectives, and to 
the lower power ones it has been intentionally imparted to a similar degree, 
a means being thereby obtained to nearly entirely eliminate this error 
by help of suitable eye- pieces. These are made to possess an equivalent 
error of opposite sign, that is, the image formed by the red rays is greater 
than that corresponding to the blue rays. Such eye -pieces therefore serve to 
compensate the unequal magnification of different colours and the images ap])ear 
free from colour up to the margin of the field. 
This compensatory action of the eye-pieces is manifested, particularly in the 
higher numbers where the limiting diaphragm is placed outside the lenses, by the 
fact that the edge of this diaphragm shows a red border, whilst the 
image of the object formed at this very edge of the diaphragm is 
perfectly colourless. 
The setting- of the eye -pieces is so arranged that the lower focal point 
of all numbers in each series lies in the same plane when inserted in the body- 
tube. No alteration of focus is therefore required on changing the eye -piece, 
and the ''optical tube -length" (i. e. the distance between the upper focal point 
of the objective and the lower one of the eye-piece), which is the standard factor 
for the magnifying power, remains constant. This optical tube -length in the 
continental microscopes (excluding small differences between the various objectives) 
is 180 mm, provided that tlie length of the body, from the screw -collar of the 
objective to the upper end of the tube on which the eye-pieces rest, is 100 mm. 
The eye -pieces of extremely low power designated as Searchers serve the 
purpose of reducing to its lowest limits the available magnification with each 
objective, thereby facilitating the preliminary examination and the 
labour of searching for particular points with high powers. Thus No. 1 
of this series enables an objective to be employed with its own 
