Nomarski
Differential Interference-Contrast Microscopy
Part IV : Applications
By W alter l ang
l aborato ry for Mi cr oscopy
CARl ZEISS. überkochen
In the comp re hen si ve de scription of Nomar­
ski differ ent ia l int erfe rence-co nt ra st (DIC)
microscopy, th e fundam entals and ex peri­
me ntal designs (Part I) and the fo rm ation of
the interference imag e (Part 11) were dis­
cu ss ed and a comparison made with the
phase-con tr ast techn ique (Pa rt 111) [26. 27.
28]. The pres ent Part IV will now give a
review of t he app lic at ions of Nom arski DIC
mic roscopy to get her w ith a list of refer­
en ces . N o c lai m is made wi t h regard to the
completene ss of the b ibl iographic data in
v iew of t he rap idl y increas ing number of
appl ications an d pu blicat io ns on the subject.
The bibliograp hy is th ere fo re on ly intended
to se rv e as a gu ide to the potential uses of
th e No marski DIC method in the different
field s o f micro scopy o f organi c and ino r­
gan ic objects.
A. Microscopy of organic objects
An e ssential advantage of Nomarski DIC
microscopy ls the fa ct that - like pha se
contra st, for example - it allows the ex­
ami natio n of uns ta ined specimens. A new
and extreme ly useful ai d has th us been
created, ab ov e all for exam in ing liv ing spec l­
men s unde r t he opti cal mic roscope .
Cytology
Us ing a liv ing cell of Haemanthus katherinae
in the pr ocess of div ision as an example,
Bajer and All en [5] demonstrate the superi­
ority of the DIC image over phase-contrast
re pr esentation: wh ile in phase contrast the
halo effe ct makes lt impossible to recognize
detarls, the spi ndi e fibers can be clearly
seen by the Nomars k i d ifferentia l interf er­
ence-cont rast met hod .
DIC micrographs of heia cells in a nutrient
so lution and denatured w ith 96 % alcohol
we re publi sh ed by Gabler and Herzog [18 .19] .
Wun derer and W itt e [47] publ ished a com­
pa rison o f photom ic rog raphs of cells and
group s of cells from the mucuous membrane
of the hum an stomach, tak en by ph ase con­
tra st and by Nomarski differential interfer­
enc e contrast. Th ese examp les prove that
th e tw o met hods co mplement each other
very nicely. However, in the case of a group
of gland ula r cells of t he gastric mucosa , the
int erference method offers the advantage of
improved detail defin it ion.
G ivin g many pr actica l exarnples, Padawer
[39] expla ins the cha racteri sti cs and advan­
tages of D IC microscopy. In one particu lar
exam pl e nuclei and vac uol es appear in the
DIC image as depressions, wh ile highly
refractive structu re s such as eos inophile
gra nula or fatty inclusio ns seem consider­
ably ele vate d . A ls o the vacuoles of macro­
p hages appear as depress lons, wh ile the
nuclear memb rane shows up as a bulge. The
author shows that phase structures located
22
outs ide the f oca l pla ne cannot always be
neg lected in th e interp reta tio n of DIC im­
ages. W hen erythrocytes are v iewed by
phase cont rast, the f ormatio n of haloes is
rather troublesom e. In th is case, th e DIC
image is unm istakab ly supe rio r. The si tu­
ation is simila r w ith epi the lia l ce lls of the
mucous membrane of the human mouth.
Duitschaever [14] uses th e DIC method fo r
micro scop ic investi g ati ons on somatic cells
in cow's milk and other body flu ids. Engel s
and Ribbe rt [15] also use Nomarsk i d iffer­
ent ial interference con trast fo r the exami­
nation of nucleoli in M usca d ome st ica. Rib ­
bert and Bier [41] mak e use of th e Nomars ki
meth od for stu dy ing inse ct ovaries .
StoII · and Gundla ch [43] compare the
pha se-contr ast im age w ith the DIC ima ge of
a cell sm ear in a sa line so lut ion . Th e l iv ing
trich omonad beside an ep ithel ial cell and
erythrocyt es shows mo re detail in interf er­
en ce co ntra st. Thi s ho lds tru e above all fo r
the marg inal portions of the trichomonad
wh ich in ph ase con trast reve al consi derable
fl ar e du e to ha lati on . A s im ilar situ at ion is
encountered in the cell smear in a sal i ne
solu t ion. Th is examp le also shows t hat it is
eas ier to d istinguish superimposed stru c­
tures in the DIC image than in phase con­
trast. The au thors prove that in such a cas e
it is frequent ly imposs ib le to re cog nize the
borders of t he ce ll in p hase contrast du e to
halatlon.
Bo tany
The ph ase -contrast technique ls weil suited
for examin ing sm all particles - especia lly
organelles - in protoplasm [44]. However,
the great depth of f le ld of th is me thod is
a dis advantage in botany . As a re sult, pha se
structures in the light path will impair th e
phase image even if they are located ou t­
side t he fo ca l plane [44] . Accord ing to Ur'
and Gabler [44]. the shall ower depth of
field of Nomarski DIC microscopy opens up
a cons iderably wider field of app lication f or
light microscopy in botany. These authors
show, amo ng others, DIC micrographs of
the ins ide and outside epidermis of Al lium
cepa, cells of Closterium lunu la and - in a
comparison w ith phase contrast - M icraste­
rias denticulata and Closterium lunula . In
the case of Allium cepa , mltochondrta, the
Golgi comp lex, leucoplasts, the nucleolus
and large and sma ll spherosomes stand ou t
in high contrast, the latter due precisely to
the great difference between their own re­
fractive index and that of the surround ing
areas,
Accord ing to Padawer [39], observation of
plant material offers considerable difficulty,
be it in ph ase contr ast or differential inter­
ference contrast. In the one case , the pro­
nou nced differe nce o f refra ct iv e index give s
ris e to heav y ha lat ion , in the ot he r b ir e­
fr ingen t compon ent s disturb the imag e. This
ha s be en proved for ex ample in the ease
of dried poll en, such as Sa lix d isco lor and
ab ove all Coreopsi s. S im ilar cond it ion s are
encoun tered w ith freshwater Chlorophyceae.
M aguir e [29] inv estigates subch rom at id stru c­
tu res in corn w ith th e aid of the No mars ki
method and, for compa ri son , in phase con­
trast.
Us ing the African blo od li ly , Ha emanth us
k atha ri nae, as an example, Allen , Dev id and
No tnerski [3] sh ow tha t the spin die fibers of
a liv ing cell during div is ion sta nd out clearl y
in the DIC imag e (see also [5] and [6]) ,
wh ereas they are hardly v isible by any other
microsc opic techn iq ues. Baum [8] uses DIC
micro sc opy to sh ow na tu ral hyb rids of
Av ena sat iva and Aven a fatu a in th e cu lti­
vat ed oa t.
Histo logy
Gab/er and Herzog [18, 19] show the thyroid
gl and of a mous e in po sit ive and negati ve
pha se co ntr ast as w eil as in Noma rski dif­
ferent ia l inte rference con trast. Nomarski
DIC is also su ited for ampl itude stain ing of
stained speci mens . as shown by Allen ,
Dav id and Notnerski [3] on large chromo­
somes of Drosophila melanogaster. Even
hu man chrom osome s ca n ea sily be examined
by the DIC meth od using amp litude sta in ing.
Very th ick bone sections gen eral ly used,
for exarnple, for examination by inci dent
f1uores cent ill urninati on, re sult in a notice­
abl e de cre ase in contr ast in the D IC lrna pe,
same as in ph ase contrast. Th is is due to the
f act t hat pr oper imag ing of t he contrast­
p roduc ing components , such as annular dia­
phragm on ph ase p late or auxiliary pr ism
on princ ipa l p rism , is no lo ng er gu aran teed.
This is d emo nstrated by Lang [28] both for
ph ase contrast and Nomars ki differential
interfer enc e contras t on th e ex am ple o f an
excessively
thick transparent specimen
(pol ished bone sec t ion) and a th in , well ­
sui ted transparent spe cime n (r at' s to ngue,
unstained) . Despite th is qu allficat lon , t he
No mars ki DIC rnet hod , owin g to its high
useful aperture and the consequent small
depth of fie ld, ls the idea l method fo r the
observation of so -calied op t ic al sections .
Th is may be ve rlf ied by an ex ample from
zo ology: Fig . 1 sho w s photo microg rap hs of
Ma cronyssus b acoti in br igh t-f ield (a) . ph ase
contrast (b) and Nomarski differential inter­
ference contrast (c) all with the same fo cal
• See the monograph meanwh i le pub ll shed by Peter
StoII : Gyne c olog lca l V ital Cytoloq y, Spr inger-Verlag
Berl tn, Heldelberg, New York 1969. whic h contal ns
numerous pracllcal examples of DIC mtcroscopy,
often with comparatl ve phase-contrast mtcrc qraphs .
plane. Figs. d and e also show the DIC
image of the same object with two other
focal plane settings.
The Nomarski DIC method makes it con­
siderably easier to analyse the structure of
thin seetions as is shown by the contrast
with the bright-field view in Fig. 2. For this
type of work the DIC method can also be
successfully applied with stained specimens.
The colour distortion caused by the No­
marski method in such cases remains within
reasonable limits. It should also be remem­
bered that the transition to bright-field ob­
servation for comparison purposes, for
instance by removing the interference con­
trast siide from the light path, is swift and
convenient.
Hematology
With the aid of Nomarski DIC microscopy,
unstained erythrocytes can be rendered
visible with excellent results (Gabler and
Herzog [18, 19]). According to the authors,
the DIC image of a crystal in the blood
Iymph of an eel is superior to the corre­
sponding phase-contrast image that ls im­
paired by halation.
Padawer [39] discusses differences between
phase-contrast and differential interference­
contrast observation of the hemolysis of
frog erythrocytes. The photomicrographs
taken under identical condltlons show non­
hemolyzed cells, spherocytes and completely
hemolyzed cells. The author shows that with
normal cells the nucleus in the DIC image
stands out more c1early from the cytoplasm
than in phase contrast. The nucleus becomes
more elevated from the cytoplasm all the
more clearly the more water the cell ab­
sorbs and the more hemoglobin it loses.
With completely hemolyzed cells the cyto­
plasm will show up only weakly due to
the loss of hemoglobin, while the nucleus
stands out in good contrast. In another case,
viz. a fresh blood smear, the coiling makes
phase-contrast observation impossible due
to heavy halation. In the DIC image, how­
ever, sufficient detail can be recognized in
spite of the stratification. In phase contrast,
fibrin fibers may appear dark or bright,
depending on whether or not they Iie in
the focal plane. This complication does not
exlst in differential interference contrast.
Neurology
Neuhoff [31] uses Nomarski DIC microscopy
to render human ganglion cells visible and
especially for examin ing cells in which an
appendix leads back to the same cell, so­
called feedback neurons.
Bacteriology
With bacteria specimens, the disturbing
halation known from phase-contrast images
presents an advantage in Nomarski DIC
microscopy; as an example, Gabler and Her­
zog [18 , 19] show a smear of Klebsiella.
Hydrobiology
Quite a number of authors have published
DIC photomicrographs of dlatorns which
show up 3-dimensionally in the DIC image.
Due to the excellent resolution of the No­
marski DIC method, minute detail can be
recognized in the diatoms. Gab/er and Her­
zog [18, 19] show the DIC image of Auliscus
sculptus. The use of Nomarski DIC micro­
scopy in micropaleontology is described by
Barbieri and MazzoJa [7] .
Padawer [39] compares phase-contrast and
differential interference-contrast images of
various diatoms. In this comparison , the
superiority of the DIC image is very evident.
In a very comprehensive paper, Allen, David,
Hirsch and Watters [2, 13] cover the subject
of image interpretation in transmitted -light
polarizing interference microscopes both of
the image-duplication and the differential
type . In addition to an extensive comparative
discussion of theoretical and experimental
principles, the differences are illustrated
impressively by a number of practical ex­
amples. Under Nomarski even large path
differences of up to 2 'I. ). between Stauro­
neis acuta diatoms and the mounting medium
give images that are rich in detail. The
Surirella robusta diatom can be reproduced
with good contrast even with an i1luminating
aperture of 1.25. As a result, object details
that are invisible at a smaller numerical
aperture of, for example , 0.6, can be clearly
distinguished.
Allen, David and Nomarski [3] report on the
fundamentals, design, function and charac­
teristics of ZEISS differential interference­
contrast equipment. A number of practical
examples explaining the special features of
the equipment concern diatoms: Staureonis
acuta diatoms in the DIC image as com­
pared to the interference-contrast image
(photographed with the ZEISS Jamin-Lebe­
deff system) show that particularly pro­
nounced gradients of optical thickness in
the specimen are reproduced very clearly in
the DIC image. The azimuth effect of the
technique can be demonstrated very impres­
sively in the Hantzschia amphioxys diatom.
Radial structures such as Anachnodicus
ehrenbergii diatoms also reveal the azimuth
effect. Using the example of the Surirella
robusta diatom, the authors explain the ad­
vantage of DIC equipment over bright-field
and phase-contrast observation, in that ex­
cellent contrast is obtained even at high
aperture. In other words, the Nomarski DIC
method has the effect of a filter that arnpli­
fies high spatial frequencies and subdues
low ones . The advantage of the shallow
depth of field of the DIC method as corn­
pared to phase contrast is i1lustrated by a
Triceratlum favus diatom.
B. Microscopy of inorganic objects
Metallography
As early as 1954, Nomarski and Mme WeiJl
[32] pointed out the advantages of differ­
ent ial interference-contrast microscopy in
the field of metallography (e, g., electro­
polished cobalt). A second publicatlon by
the same authors [33] dealt exclusively with
metallographic applications. Among other
things, it was devoted to a detailed study of
various growth spirals of silicon carbide
(SiC). Nomarski and Mme WeiJl were able
to prove that growth steps of 440 A ± 30 A
for example, can be resolved without dlf­
ficulty. Under certain conditions, arelief of
the order of a few Angsträm units can be
recognized in the DIC image. Slipbands in
cobalt subjected to a tension of 440 g/mm 2
for aperiod of five minutes can be repro­
duced with excellent contrast. The photo­
micrographs show various patterns of slip­
bands which, with bright-field illumination,
for instance, can be recogn ized only with
difficulty or not at all.
Measuring thin films of an order of magni­
tude of 2000 A with the aid of yellow sodium
light (589 ,u m) and Nomarski differential
interference equipment, Le Mehaule [24] ob ­
tains an accuracy of ± 1.5 %. The author
comes to the conclusion that the DIC
method is superior to bright-field and par­
ticularly to phase-contrast observation for
testing highly polished surfaces, exam ining
thin films on glass substrates (vacuum­
deposited films), checking quenched steel
for undesirable phases such as ferrites,
austenites, etc. , and finally for the testing
of diffusion processes by phase changes,
the creation of new phases, recrystallization
or other defects such as porosity due to
different rates of diffuston between two
elements, and lastly checking for dis­
locations and displacement of grain bound­
aries. As practical examples Le Mehaule
publishes photomicrographs of quenched
Cr-Ni-Mo steel, cold-worked bronze and
sintered iron .
Bertocci and Noggle [9] use a differential
interference-contrast system for the quanti­
tative examination of small etched copper
surfaces down to a mean size of 6.um . De­
pending on the magnification of the objec­
tive used , they attain an accuracy of be­
tween ± 5' and ± 30' .
The superiority of Nomarski DIC micros­
copy over bright-field observation is illus­
trated by Gahm [20] who compares photo­
micrographs of unalloyed quenched and
tempered steel taken by both techniques.
The deformation of the surrounding area
produced by a microhardness indenter, of
which no trace ts visible in bright field, can
be c1early dislinguished in the DIC image.
At the same time, the series of micrographs
shows the azimuth effect characteristic of
the Nomarski DIC method, which can here
be observed along a linear grinding trace.
leg/itsch and Mitsehe [23] use Nomarski to
investigate the metallographic structure of
Vacutherm sampies (Widmannstätten struc­
tures, ferrite after yla-conversion, pearlite
containing 0.85 % C), of low-temperature
martensite, white cast iron (hypereutectic
and hardened) and fused high-speed steel.
On the left, Fig . 3 shows a martensite
needle magnified 800 x in bright fleld, on
the right, the same needle in Nomarski dif­
ferential interference contrast.
Crystallography
In 1954 Nomarski and Mme WeilJ [32] re­
ported on numerous practical examples and
illustrated the use of Nomarski DIC micros­
copy in crystallography, e. g. growth spirals
on silicon carbide (SiC) with triangular
23
la
Ib
lc
ld
1e
Fig. 1: Ma cr onyssus bacoti i n bri ght field (a), in phase co ntrast (b), and
different ial int erference co ntr ast (c , d , e). Figs . d and e w ere taken with different
focal plane se tt ings. ZEI SS U ltr aphot 11, Piana ch rom at 40 x , 0.65 N .A.,
magnifi catlon 320 x .
24
2a
2b
5a
3a
3b
5b
4a
4b
Fig . 2: C at mus eie in bright field (a) and in Nom arski
DIC (b). ZEISS Ultraphot 11, Pl anach r omat 16 x,
0.35 N . A ., mag nifi cati on 144 x .
Fig . 3: M artensite ne edl e, bright field (a), Nom arski
D IC (b). ZEISS Ultraphot 11, Ep iplan 100 x, 1.25 N . A.,
oi I, magnificati on 800 x .
Fig . 4: Wafer: bright field (Iett) and Nomarski DIC
mic rogr aph (right). Z EISS Ultraphot 11 , Epiplan 8 x ,
0.2 N . A ., mag nificat i on 72 x .
Fig . 5: Crcss-aec t ton o f an inte gr ated c irc uit that
failed d ue to ov er lo ad , i n bright field (a) and in
Nomarsk i DIC (b). ZEISS Ultraphot 11 , Epiplan 8 x,
0.2 N . A ., magnifi cati on 72.
25
symmetry; principal spirals with secondary
recrystallization on SiC; star-shaped growth
spirals etc .
Using the example of microhardness Inden­
tations in cleavage surfaces of sodium­
chloride crystals and potassium-chloride
crystals, Gahm [20] shows that DIC micros­
copy can be used to advantage for quanti­
tative investigations. Slipbands that cannot
be recognized under bright-field illumination
stand out with extraordinary c1arity in the
DIC image.
The superiority of the Nomarski DIC method
over phase contrast is clearly evident from
the replica of a calcite cleavage surface
[18, 19].
Padawer [39] compares sodium-chloride
crystals in bright field , phase contrast and
differential interference centrast. In bright
field practically only the contours of the
crystals will become visible, even if the
illuminating aperture ls reduced , and in the
phase-contrast image, large parts of the ob­
j ect will be ve iled by halation .
For the optical staining and examination of
the surface of germanium and sili con oxide,
Franeon [17] takes recourse to reflected­
light differential interference centrast. How­
ever, ammonium-alum crystals can be repro­
duced with a wealth of detail by optical
stain ing eve n when transmitted light is used.
Besides a comprehensive and easily under­
standable introduction to Nomarski DIC
microscopy, Franr;on [17] publishes a num­
ber of photomicrographs of a germanium
surface , a microcircuit and a cadmium­
telluride f ilm on silicon dioxide.
Owing to its high resolution , the Nomarski
method is weil suited for the examination of
silicon monocrystals, as has been shown by
Vieweg-Gutberlet [45] . If the specimens are
properly etched, inhomogeneities in the
doping concentrations such as strtattons,
the microstructures of strlatlons. stacking
faults in concentrations etc. can be made
visible. Fig . 4 shows a wafer in bright field
(left) and in Nomarski differential interfer­
ence contrast (right).
Fig . 5 shows a cross-section of an integrated
ci rcuit that failed due to overload, once in
bright fleld (a) and on ce in DIC (b). Differen­
tial interference contrast above all repro­
duces the conductors with greater detail.
Glass technology
Minute detalls in a glass surface are re­
produced with high contrast in the DIC
image . As is proved by Gabler and Herzog
[18, 19], pronounced differences of refrac­ tive inde x between object and mounting
med ium wh ich, in phase contrast, result in
extrem ely disturblng haloes, do not have the
same unfavorable effect in the DIC image.
Mineralogy
Gahm [20] has suc cessfully used Nomarski
DIC mi croscopy to make microhardness
impressions in varl ous minerals (e. g. covel­
lite, boul angerite) visible. Cracks, scab­
biness and bulges around a microhardness
indentation in a periclase cleavage surface
can be made optimally vis ibl e by color con­
trast. Von Gehl en and Piller [21] have shown
that Nomarski DIC microscopy is an ideal
means for exam in ing polished specimens or
ore minerals for hardness differences. This
method has pro ved to be superior to the
conv entional "Schneiderhöhn llne" , More­
over, the Nomarski method is a valuable aid
in te sting the quality of polished surfaces
whose reflec tivi ty is to be measured by
microphotometry . Finally, with sub-stage il­
lumination the Nomarski method offers many
advantages for assessing the morphographic
properties of fine-grain minerals and iden­
tifying clay minerals , as Correns and Piller
[11] have show n.
Semiconductor technoiogy
In the introduction to his paper, Le Mehaute
[24] describes th e fundamentals and charac­
teristics of the Nomarski differential inter­
feren ce- cont ras t method and conti nues by
giving a summary of its advantages over
bright-field and phas e-co nt rast obs erv ation,
particularly for metallographi c uses. In the
semicondu ctor f ield the DIC technique may
be used to advantage for observing struc­
tural chang es such as phase transit ions, the
formation of new phases, recrystallization
processes, et c . Le M ehaute shows that th e
DIC image of a transistor reveals far more
deta il than wou/d a bright-field image.
26
Wh en examining oriented linear phase struc­ tures, their orientation relat ive to the split­ ting direction of the Nomarski prisms must
be taken into account [28]. The phase-con­ trast techn ique has no inherent azimuth
effect. However, it has the disadvantage of
greater depth of field so that phase ob jects
Iying outside the focal plane will appear in
the image, for example, as disturbing dlf­
fra ction fringes.
Plastics
Like clear transparent glas s, cle ar trans­ parent plastlos are ideal phase objects.
Ac cording to Gahm [20], microhardness
impressions of extremely minute irregu­ larities that are invisible in br ight field can
be emphasized by suitable setting of the
background , above all in color contrast.
A comparison between the opt ical staining
of spherical plastic parts in dark field and
differential interference contrast is made by
Padawer [39] . This comparison proves the
advantages of the DIC method by which
even very small particles are clearly repro­ duced beside larger objects. With the aid
of polystyrene spheres with a mean diameter
of 1.3 /lm , mounted in glycerin , Padawer
demonstrates the dependence of the DIC
image on the illuminating aperture. The
optimum illuminating aperture depends both
on the object and the illum inating aperture.
In the spec lal ca se under discussion, a set­ ting of 75 % of the maximum aperture has
proved particularly favorable. In addition ,
with the aid of two photomicrographs, the
author explains the effect of defocusing on
the DIC image. A number of micrographs of
plastic spheres shows that diffusion proces­
ses within the particles, inv olv ing a variation
of optical thickness, can be clearly repro­
du ced by differential interference contrast.
Literature (up to February 1970)
[1] Allen, R. 0., G. B . Davld, and L. F. Hir sh ir .:
Proc . R. rnlcr. So c. 1 (1966) 141
[2] Allen, R. 0 ., G. B. Davld, L. F. Hirsh ir ., and
C. D. Watters: I. Contrast Generators in M odi­
l ied Polarizlng Mlcroscopes and the Sou rces o f
Contrast. Resear ch report.
[3] Allen, R. 0 .. G_ B. Dav ld , and G. N omarski : Z .
w ls s. M lkr. 69 (1969) 193
[4] Bai er , A.: 1. Cell. Bio l. 27 (1965) 7A
[5] Beter , A, and R. D . Allen : Science, N. Y. 151
(1966) 572
[6J Baier, A., and R. D. Allen: J. Cell. Scl. 1 (1966)
455
[7] Barbieri , F., and S . M azzola : Acta Naturali a IV
(1968) Fasc . 2
[8J Baum , B. R.: Can adl an Journal of Bot any 47
( 1969) 85-91
[9] Berto cci, U.. and T. S . Nog gle : Rev. Se I. Instr .
37 (1966) 1750
[10] Bes sts. M., and J. P. Thl ery : Revue Hernatol . 12
(1957) 518
[11] Correns , C. W., and H. Piller: Hdb. d. Mikro­
skopie in der Te chn ik , Va l . 4, Part 1, Ed. H.
Fre und . 2nd editio n (In prep .) , Urns chau-Verl aq
Frankfurt
[12] Davld. G. B ., R. D . Allen, L. F. Hir sh Ir .. and
C . D . Watters: Pro c . R. mi cr . Soc . 1 (1966) 142
(13] De vtti , G. B .. R. D. Allen, L. F. Hir sh jr .. C. D .
Walters: 11. The Instrumental Ext ln ct lon Fact ar.
Th e Control of Contrast. and l. lrn tt s o f Sens it iv ity .
Res earch report
[14] Du itschaever, C. L.: Mikroskopie 23 (1968) 345
[15] Engel s , W., and D. Rlbbert: Experlentia 25 (1969)
[16J Everin gham , J. W :: An at. Rec . 157 (1967) 242
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