412 J. J. Woodward—Nomenclature of Achromatic 
conveniently with all powers from the three inch upward. J 
o not, however, insist on any particular distance, but only that 
the distance selected shall be stated in each case until some uni- 
form plan shall be generally agreed upon. 
In this connection I may add that the actual plan of the Con- 
tinental opticians is also unsatisfactory. In the matter of angle 
of aperture, when they give it at all, they also, as a rule, give only 
the maximum. In the matter of magnifying power, when they 
give any information, they attempt, as a rule, to give the 
nifying power of the objective with each eye-piece as acta 
looked through when in use. But as the magnifying power of 
the combination under these circumstances involves the dis 
tance of distinct vision for each observer, it is evident that the 
figures thus furnished cannot have any practical value. 
Pending the adoption of some such system as I have sug: 
gested above, it will be necessary for the microscopist to meas 
ure for himself the magnifying power of the objectives he uses, 
whenever he desires to be possessed of the real information con- 
Multiply the distance between the two micrometers i” the 
and decimals, by the magnifying power, and divide a be 
Square of the magnifying power pl one ; the result ie 
n us = 
the equivalent focal length (for the given conditions) 12 
inch. 
ee es 
