69 
Objectives for the Compound Microscope. 
different in different objectives. I select a few from my note-book 
in illustration, purposely omitting to name the makers : — 
Magnifying power 
at uncovered. 
Magnifying power 
at covered. 
No. 1 .. .. 
200 
225 
No. 2 .. .. 
250 
275 
No. 3 .. .. 
300 
333 
No. 4 
350 
500 
No. 5 .. .. 
570 
630 
No. 6 .. .. 
900 
1100 
No. 7 .. .. 
975 
1180 
No. 8 .. .. 
890 
1250 
These values were obtained by throwing the image of a micro- 
meter on a cardboard screen, using the objective without an eye- 
piece, and the distance from micrometer to screen in each case 
remained the same, the screw-collar and the focal adjustment of 
the objective being modified. Of course, for all intermediate positions 
of the screw-collar, intermediate values result. The distance used 
was 48 inches in some of the cases, 50 in the others. With shorter 
distances, the amount of the difference is diminished, but its ratio 
to the magnifying power at uncovered is not materially changed, 
as anyone can convince himself by comparing a stage micrometer, 
as seen by any corrected objective, with an eye-piece micrometer, 
first at uncovered and then with the full correction for cover. 
Now it is evident that even if, by the formula of Mr. Cross, or 
otherwise, we could obtain accurate equivalent focal lengths for any 
one position of the cover correction, the result would not be true 
for any other position of the cover correction. 
We are told by a recent writer that the practice of the opticians 
is to name the combination at its performance uncovered, that is, at 
precisely the adjustment least used. If, however, scientific accuracy 
in the matter is desirable for any purpose whatever, it is evident 
from the above that at least the maximum and minimum should be 
furnished by the maker. 
It is also evident that the considerations here offered, aside from 
their bearing on the nomenclature of objectives, have a high 
practical value to all those who attempt to make micrometrio 
measurements with modern high-power objectives ; for the practice 
recommended in the text-books, and too generally pursued, is to 
give values to an eye-piece micrometer by comparing it with a 
stage micrometer at a fixed position of the draw-tube, and to use 
these values in subsequent measurements. Now as these values 
vary considerably with the cover correction, it is to be feared that 
the majority of the measurements made with high powers during 
the last twenty years are sadly inaccurate. 
