100 NOBERT’S TEST PLATE. 
he will find this difficulty almost insurmountable. In the 
microscope the micrometer is an aid in counting, but in 
counting lines of such exquisite fineness, either the microm- 
eter or the stage must be moved, and it is next to impossible 
to construct apparatus that can be moved at once qovooo of | 
an inch and no more. It would require the genius and skill — 
of Nobert himself to do it. sa 
These trials show conclusively, that it is not the great 
power of the objective that is important (for in many of the 
trials here reported the lower powers have given the best 
results, and the Tolles 4 immersion the best on record), but 
it is the skill of the optician in making the instrument. 1 
have since tried the Wales’ objective dey and resolved the 
13th band well,—thus doing what Mr. G. did with itin 
water; the inference must be that Mr. G. did not obtain its 
best work. 

he 
hain — Since the foregoing was written, Dr. Barnard has made more 
and I am well satisfied that he has seen the 19th band with 4 
ptt zz and Tolles’ 4, both dry objectives. si performance fairly 
maa any thing yet peia either in this country or Europe. Dr 
arnard writes (Jan. 29), that he found that the anvi of the lines was _ 
eae with the very difficulties referred to above, in addition to which 
there is another trouble, the whole width of a band is not in perfect focus — | 
at once; this necessitates a slight change of focal adjustment, and any 2 
change rehire it extremely difficult to fix,even with the cobweb microm- — 
eter, the exact line last counted. He made five counts of the 19th —_ 
with the zy, namely :— 
t nn to the English inch. 4. 106.226 to the English inch. 
b 108, “ “oe : 5. 115,474 
3. iy t “ mean, 110,820  “ z 
he number, according to Nobert, is 112,668. He counts for the 15 
discrep“ 

, 4 variation of two lines each way covers the extremes of 
ine 
Pod Greenleaf has just tried (February 7th) an immersion aren | 
es’ 7's. He resolved the 10th, 11th, and 12th bands perfectly ; the 1 
