70 Remarks on the Nomenclature of Achromatic 
The difficulties in the way of a nomenclature based upon equiva- 
lent focal lengths have been still further increased since the intro- 
duction of immersion objectives. The compound objective is usually 
furnished with two fronts, one for wet and one for dry use. Of 
these the wet usually gives the greatest magnifying power at any 
given distance. I have also measured an objective which, when 
corrected for the thickest cover through which it will work dry, is 
just corrected for uncovered wet, and by approximating the posterior 
pair of combinations still nearer to the anterior, corrects for cover 
wet, thus increasing the magnifying power when the objective is in 
use wet precisely as if two fronts were used. 
The difference in magnifying power resulting from the modifi- 
cations given to make the objective perform wet is quite considerable, 
as may be seen by the following extracts from my note-book. 
Magnifying Power. 
Dry. 
Wet. 
Uncovered. 
Covered. 
Uncovered. 
Covered. 
No. 1 . . . . 
225 
250 
250 
275 
No. 2 
425 
490 
450 
500 
No. 3 .. .. 
700 
900 
900 
1000 
No. 4 
770 
910 
900 
1100 
No. 5 
790 
930 
975 
1180 
Of these objectives No. 1 had but one front, the correction for 
wet being made by the cover correction ; the others had two separate 
fronts, one for wet, the other for dry. 
Now it is just in connection with these complex objectives, with 
double fronts or other devices to correct for wet and dry, that the 
greatest diversity of nomenclature exists. In one quarter it is 
claimed that the system should have two names, one based on its 
magnifying power at uncovered wet, the other on its magnifying 
power at uncovered dry ; in other quarters the practice has been to 
give the system but a single name, derived from the magnifying 
power at uncovered dry alone. It is evident, however, that neither 
of these plans has any pretension to scientific accuracy, and that 
if the makers will not give at least the maximum and minimum 
for both dry and wet, the purchaser must learn to measure for 
himself. 
A similar remark applies to the angle of aperture as stated by 
the makers, for each objective sold. As a rule they give the greatest 
angle attainable by the combination, which is generally — not always 
— the angle at the correction for thickest cover, their usual practice 
in this case being the reverse of their usual practice with regard to 
magnifying power. Now I have seen objectives with an angle of 
170° and upwards at full cover correction, which did not exceed 140° 
