1909.] NATURAL SCIENCES OF PHILADELPHIA. 179 



previously made experiments, for when I first realized the effect that 

 the size of the illuminated aperture should have on the apparent size 

 of an object, the theoretical basis for this conclusion seemed so sound 

 that I started a series of measurements to establish a table of correc- 

 tions for "antipoint" for future use. 



Failing to get any consistent results, or even any variations beyond 

 the probable limits of instrumental error, in the course of micrometry 

 of various objects at odd times, I finally made the following measure- 

 ments with higher powers, adopting as most suitable objects two 

 well-stained human blood corpuscles, mounted in balsam, and so 

 marked that they could be found at any time, not only because they 

 represented a class of objects frequently subjected to measurement, 

 but also for the reason that they furnished an absorption image that 

 should be particularly susceptible to dioptric law. Full data are as 

 follows : 



1. April 18: Stand, Zentmayer Centennial. Objective, Zeiss 3 mm. 

 Apochromat, 1.40 N. A. Condenser, Powell & Lealand Apochromat, 

 1.40 N. A., no diaphragm, focussed for greatest aplanatic aperture, 

 about 1.20 N. A. Stage Micrometer, Rogers "A" Division, third one- 

 thousandth inch, Zentmayer Filar Micrometer. Diameter of corpuscle 

 No. 1, .00032050 inch; diameter of corpuscle No. 2, .00031745 inch. 



2. April 18: Everything exactly as above, except that a diaphragm 

 was inserted in the condenser, reducing the cone of illumination to 

 .20 N. A. Diameter of corpuscle No. 1, .00031S60 inch; diameter of 

 corpuscle No. 2, .00031747 inch. Variation from first measurements, 

 corpuscle No. 1, — .00000 190 Jnch; corpuscle No. 2, +.00000002 inch. 



3. April 19: Stand, Zentmayer Centennial. Objective, Bausch & 

 Lomb one-eighth inch, .85 N. A. Condenser, Beck Achromatic, 1.00 

 N. A., diaphragmed to .20 N. A. Stage Micrometer, Rogers "A" 

 Division, third one-thousandth inch, Zentmayer Filar Micrometer. 

 Diameter of corpuscle No. 1, .00031890 inch; diameter of corpuscle 

 No. 2, .00031665. Variation from first measurements, corpuscle No. 1, 

 —.0000016 inch; corpuscle No. 2, —.00000080 inch. 



4. April 19: Stand, Watson Van Heurck. Objective, Beck one- 

 eighth .90 N. A. Condenser, Bausch & Lomb Abbe Achromatic 

 1.00 N. A. diaphragmed to .50 N. A. Stage Micrometer, Fasoldt 

 "A" Division, second one-thousandth, Bausch & Lomb Filar Micro- 

 meter. Measurement of corpuscle No. 1, .00031609 inch. Variation 

 from first measurement, — .00000441. 



5. April 28: Stand, Zentmayer Centennial. Objective, Bausch & 

 Lomb one-twelfth inch, W. I., 1.00 N. A. Condenser, Beck Achroma- 



