390 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
what similar error of optical excentricity, the image observed by the 
eye-piece when not in the optic axis, being formed not by the central 
rays from the objective, but by the marginal rays. To obviate this 
Fig. 43. 
Prof. Abbe (see figs. 43 and 44) while making use of Mold’s device for 
moving the eye-piece across the field of view, added a second lens close 
it can be made to traverse the whole of the fixed microscopic image. It is im- 
portant to liave some means of readily bringing the eye-piece back to the axis of 
the Microscope, and of noting tlie positions of the micrometer and eye-piece slides 
which correspond to this adjustment. For this purpose three diaphragms were 
employed, each perforated with a minute hole ; one is placed over the objective, one 
in the middle of the body-tube, and the third immediately under the eye-piece, so 
that they only transmit rays along the axis of the tube ; by this contrivance the 
eye-piece can be adjusted in the axis by bringing it into the position in which the 
cross-wires exactly divide the small circular openings of the diapliragm into four 
equal quadrants; the position is then noted by marking an index line upon the 
eye-piece slide and one of its guides, and reading the micrometer-screw, so that it 
may at any time be at once recovered without a fresh adjustment. If it is required 
to use another part of the screw for measuring purposes it is only necessary to adjust 
the eye-piece to the axis, note the position of the cross-wires upon an object, and 
then after moving the screw to the desired point, to bring back the cross-wires to the 
same position by means of the eye-piece slide. In attempting to make a preliminary 
series of measurements Mohl found that in spite of the solid stand employed in his 
instrument and the massive plate to which the tube and micrometer were fixed, 
the pressure of the hand upon the micrometer-screw produced such considerable 
deflections in the instrument that it was impossible to make accurate adjustments, 
and it was only by fixing the tube to the stand by means of a rectangular framework 
of brass plates that it could be made sufficiently rigid.” 
