1902] MICROSCOPICAL JOURNAL. 117 
mm. apochromat of measured N.A. ‘985 employed in 
conjunction with a copper acetate screen, the transverse 
strie of a balsam mounted Amphipleura pellucida, run- 
ning at about 94,000 to the inch, have been certainly 
seen and held for short intervals, 95,446 lines per inch 
being, according to Abbe, the theoretical separating lim- 
it for N.A. ‘99 with oblique white light. When the il- 
luminating cone was cut down soas to afford a working 
aperture of much under ‘821, no true resolution could be 
distinguished. The effect of stopping out a small central 
portion of the large cone was also tried, the valve being 
then found to exhibit a smudgy corrugated appearance, 
but no cleanly-separated striation. Thus it would seem 
that the complete large solid cone is practically absolutely 
essential to the formation of a true image of an object 
possessing fine structure, or at least of the nearest pos- 
sible approximation to such that the N.A. of the objec- 
tive employed will allow. It must not be thought that 
this maximum resolution was attained or is attainable by 
merely screwing a fairly good objective on the nosepiece 
of any microscope stand provided with a condenser cap- 
able of yielding a large aplanatic cone. Other most im- 
portant conditions are essential to success. Many years 
ago it was found necessary, in order to properly appre- 
ciate the faint details of images produced by means of a 
large illuminating cone, to work in a well-darkened 
room. Daylight must be excluded, and the microscope 
lamp should be provided with the well-known metal 
chimney, into the front of which a glass slip is inserted. 
An additional (small), well-shaded lamp is admissible 
when required for sketching or reference purposes, and 
will prove useful for reading the stage finder. In the 
second place, it will be found most advantageous in very 
difficult observations, after the preparation has been ar- 
ranged on the stage and all optical adjustments carefully 
made, to completely rest the eye for an interval of from 
