Objectives for the Compound Microscope. 409 
Magnifying power Magnifying power 
at uncovered. at covered. 
Nald pees vik 990 canis age 
No. 2, - - - 250 - - - 275 
Nei Opi a6) se Soe 800) o ieee pan aee 
Dee: Seip sete gat 350 von: (ucgdeicelees gk 
NOB e: escent esiot OD) teeter 
DO Be ict epic nsPics meth te MOO nati ae ee a 
1. Ck eee erimaaiens Caner Seems ae FS 
No. - - - 890 - - 1250 
These values were obtained by throwing the image of a 
micrometer on a card board screen, using the objective without 
an eye-piece, and the distance from micrometer to screen in eac 
ease remained the same, the screw collar and the focal adjust- 
ment of the objective being modified. Of course, for all inter- 
mediate positions of the screw collar intermediate values result. 
‘alge ae ver. 
Now it is evident that even if, by the formula of Mr. Cross, 
or otherwise, we could obtain accurate equivalent focal lengths 
wer objectives ; for 
s, and too generally 
pursued, is to give values to an eye-plece micrometer by com- 
am it with a stage micrometer at a fixed position of the 
w tube, and to use these values in subsequent measurements. 
