ee 
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Dr. M. C. White, Diffraction in Microscopic Vision. 383 
cut. In the 28d band on the Nobert’s test, employed gee 
writer, with a moderate power irregular lines are seen, indicat- 
ing that the depth of the lines is 
not uniform but more like the 
j 5. 
opake lines 1—6 seen in section -w-@-@ @- © @- @-™— 
viewing the first band of No- a Sot Pee oe speak 
bert’s test, the focus can be so i ry a: 4 a 
adjusted that each line appears 
like two dark lines. ; 
In this case there is no great danger of mistaking the number 
of the real lines, but with lines ruled much deeper for the purpose 
of experiment there is great difficulty in determining the number 
of real lines. Every observer has doubtless noticed when exam- 
ining diatoms or other delicate objects with oblique light, a fainte 
image overlying the real object. I have often been puzzled to 
tell whether these two images did not belong to opposite sides 
of the diatom. May not this phenomena be due to diffraction ? 
The sources of error here indicated may possibly help to ex- 
lai 2 particular 
the light less than a sharp edge or angle. According to Har- 
tung (“Das Mikroskop,” P. 722) Amici’s microscope of 94° ap- 
seen, and call the semi- 
e first fringe on either 
side would be y=1-21, the apparent increase of the breadth of 
Ai 
roportion t dth of the crystal, the effect of di 
Ns coke th A athe ar as pound 7 and its crystal- 
e 
