Et 
J.J. Woodward on Nobert’s Test-plate. 171 
be studied under a power of from two to six diameters. In 
counting the lines on them some doubt might arise, especially 
in the case of the 18th and 19th band, as to the real number 
of lines; for certain spurious lines, which are interference-phe- — 
nomena, may be seen'on the margin of the bands, and it is 
not always easy to tell which is the last real and which is the 
first spurious line. A comparison of several glass positives 
from different negatives with each other, and with the bands 
as seen in the microscope, where a change of focus materially 
aids in the determination, has led me to adopt the count above 
given. 
Besides the prints on glass, I send some paper prints en- 
larged to two thousand diameters. These show the lines very 
well to the naked eye, but the loss of definition, inevitable in 
enlarged paper-prints, leaves so little difference in appearance 
between the real lines, and the spurious ones, on the edges of 
the bands, that they cannot be used for a count. 
h comparing other lenses with the new immersion ;!;th of 
Powell and Lealand, thus found to excel so far their dry ;',th 
and 2.th, I obtained the following results. 
An immersion ;';th by Wales resolved the sixteenth band ; 
an Immersion ;';th by Wales resolved the seventeenth band. 
ing to Dr. Barnard. A Tolles’ immersion 3th, just construct- 
D 
immersion !th just constructed for him by Mr. Tolles, he also 
ailed to go beyond the fourteenth band. While this paper 
eturning now to this immersion th, it may be remarked 
that the work just done with it has an important bearing on 
the question of the real limits of microscopic vision. Nobert 
in sending me the plate above described, wrote me, that in his 
Opinion, the fifteenth band was the limit of possible microsco- 
pie vision. He based this opinion upon Fraunhéter’s formula 
with regard to the spectra of gratings, and upon the known 
> Give length of light undulations. Dr. Barnard of Columbia 
College, New York, after reading Nobert’s letter, writes me 
that in his opinion, Fraunhifer’ s formula does not apply to the 
