14 Tolle’s \th and Powell and Lealand’ s Immersion f^th. 
cover adjustment a little less than 4 th of an inch in equivalent focal 
length, that is, of higher power. 
Now, on counting precisely the same portion of the same frus- 
tule in the positive made with the Powell and Lealand objective (the 
portion here counted being one inch long, whereas in the other it 
was one- fifth, but in both cases exactly the same portion), I find 
just 75 lines (seventy-five lines) to the inch. 95,000 divided by 
75 gives 1266— f- diameters at 48 inches distance. 48 divided by 
1266 gives ’03874-, making the objective at this point of the 
cover adjustment a little less than a th of an inch in equivalent 
focal length. It will be seen that this differs materially from Dr. 
Woodward’s “ published measurements.” He gives in his table on 
page 29, vol. vii., 1100 diameters at full correction for thickest 
cover at 48 inches distance (yet on the glass positive which I have 
counted he has marked 1140 diam). I make it 12664- diam. 
Let us reverse the process, take the Doctor’s “ published measure- 
ments,” 1100 diam., and multiply it by 75, the number of lines 
to the inch in the positive; this willgive 82,500 as the fineness of 
the lines on the Am. pellucida which he has photographed (1140 
diam. will give 85,500), quite a falling off from his 95,000. If 
the “ published measurements ” are right, the Am. 'pellucida is not 
95,000 to the inch. 
It will be seen that instead of being about three times the power 
of the Tolies’ |th, which Dr. W. has called a little higher than a 
4 th — and my count of the photograph agrees closely with his state- 
ment — the Powell and Lealand objective is five times (very nearly) 
the power. This increase of fifty per cent, over the nominal power 
cannot be altogether ascribed to the cover adjustment, as Dr. W. in 
his table above quoted gives the entire range at less than twenty 
per cent. In fact, the objective is at least a T V»th at its uncovered 
point, and this was known to Dr. Woodward before he made the 
comparison with the Tolies’ 4 th, with the disparaging assertion that 
“ the new 4 th cannot be claimed to supersede the highest powers 
now in use, yet nevertheless is not, in my opinion, injurious to the 
4 th.” Mr. Charles Stodder writes, with good authority, probably 
under the date July 20 th, 1871 (this was nearly two months before 
I saw Dr. Woodward’s “ Note ”), “ He has now ascertained that the 
Powell and Lealand so-called |th objectives are really T yh. ; the 
so-called WiL is a T 4 th.” * This did not reach this country until 
after my “ Remarks ” had been sent to this Journal, and I knew 
nothing of Mr. Stodder ’s communication until I saw it in print, 
and he knew nothing of mine until he saw it in print. 
In the number of this Journal for April, 1872, page 166, Dr. 
Woodward alludes to a so-called 4 oth by Wm. Wales, and after, “a 
determination of its magnifying power, however, shows that un- 
* See ‘ M. M. J.,’ vol. vi., p. 203. 
