246 Extracts from Mr. H. E. Fri;pp's Translation of 
is played by diffractiye phenomena in forming the image of the 
structure in question. A suitable test-object being placed in focus, 
and the light being suitably regulated by diaphragms placed imme- 
diately ahove the 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 imme- 
diate result of experiments carried out in this manner was as follows, 
it being first premised that every trial was made with very correct 
low-power objectives (IJ to J inch) and corresponding weak ampli- 
fication : 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 beforehand, system of lines scratched in glass, 
whose linear distance varied from g Jq- ijich to two- iiich ; 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 — 
(i.) When all light separated from the incident rays by diffrac- 
tion was completely shut off by the diaphragm, so that the image 
of the preparation was formed solely by the remaining undiffracted 
rays, the sharpness of outline at the confines of the unequally trans- 
parent parts of the field was not 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 recog- 
nition of separate structural particles be sensibly hindered, provided 
that not more than 30 to 50 of such particles are found in 2V inch.* 
But the more this number is exceeded, so much the more of detail 
disappears ; so that when the fineness of detail reaches 100 parts to 
the millimeter (that is, when their interspace is only ^woo inch) 
nothing remains visible except a homogeneous surface whatever 
magnifying power be used, or whatever mode of illuminating 
(direct or oblique). Even a couple of lines ruled on a glass will, 
under the circumstances above stated, be not otherwise distinguish- 
able than as one broader line with sharp outlines. With the most 
powerful immersion lens nothing at all can be seen of the markings 
of Fleurosigma angulatum, and even the coarse lines of SijpparcTiia 
Janeira remain unrecognizable with a power of 200. In the case 
* The definition of number is here uncertain, because the exclusion of dif- 
fractcd rays, whose diffraction is slight, can only be obtained by using a diaphragm 
pierced with small openings. 
