A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 197 



aplanatic substage condensei-. It is true that under such 

 conditions diffraction effects are conspicuously absent, the picture 

 of a translucent object, such as a diatom, exhibiting almost as 

 little contrast as would a perforated plate of clear glass held at 

 arm's length between the eye and a bright background. But, 

 although the resultant image is pale, the delineating and defining 

 power of a good lens thus used is such, that in the case of 

 a 4-mm. apochromat of measured N.A. '985, employed in 

 conjunction with a copper acetate screen, the transverse striae 

 of a balsam mounted Amphipleura pelhtcida, running 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 limit for N.A. '99 with oblique white 

 light. When the illuminating cone was cut down so as 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 corr-ugated appearance, but no cleanly- 

 separated striation. Thus it would seem that the complete lai-ge 

 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 possible approximation to such that the N.A. of 

 the objective 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 to the nosepiece of any 

 microscope stand provided with a condenser capable of yielding 

 a large aplanatic oone. Other most important conditions are 

 essential to success. In the first place, Mr. Nelson, the originator 

 of critical microscopy, many years ago found it necessary, in 

 order to properly appreciate the faint details of images produced 

 by means of a large illuminating cone, to work in a well- 

 darkened room. Daylight must he 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 advan- 

 tageous in very difficult observations, after the preparation 

 has been arranged on the stage, and all optical adjustments 

 carefully made, to completely rest the eye for an interval of 



