234 ON THE THEOKY OF THE MICROSCOPE. 



objective, as closely as possible to its upper focal plane, for the 

 purpose of excluding at will one or another portion of the groups 

 of rays exhibiting diffractive effects, the image of the preparation, 

 as formed by those rays only which were not so shut off, could be 

 readily observed with the ordinary ocular. The immediate result 

 of experiments carried out in this manner was as follows, it being 

 first premised that every determinative trial was made with very 

 correct low power objectives (IJ to i inch) and corresponding weak 

 amplification: — Higher powers, an immersion lens of | inch in 

 particular being used only to control the results obtained already 

 with coarse objects by experiments on the finer diatoms. The 

 preparations for all decisive trials were of such a kind that their 

 structure was accurately known before hand, various granules of 

 finely powdered substances, system of lines scratched in glass, 

 whose linear distance varied from gi ^ inch to y-^Vo ^^^^ > ^^^^ 

 similar groups of lines ruled on silvered glass, the silver coating 

 being immeasurably thin ; groups of lines crossing each other 

 without any difference of level were obtained by laying upon 

 each other two glasses, the surface in contact being separately ruled. 



The facts thus ascertained are — 



{{.) "When all light separated from the incident rays by 

 diffraction was completely shut off by the diaphragm, so that the 

 image of the preparation was formed solely by the remaining 

 undiffractcd rays, the sharpness of outline at the confines of the 

 unequally transparent parts of the field was 7iot affected, provided 

 the opening of the diaphragm remained sufficiently large, so that no 

 diffraction arising from the reduction of its opening should occasion 

 any visible lowering of the "necessary amplification;" nor will 

 the clear recognition of separate structural particles be sensibly 

 hindered, provided that not more than 30 to 50 of such particles 

 are found in -jV inch.* But the more this number is exceeded so 

 much the more of detail disappears, so that when the fineness of 



* The defiaition of number is here uncertain, because the exclusion of 

 diffracted rays, whose diffraction is slight, can only be obtained by using a 

 diaphragm pierced with small 'openings. 



