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Note to fig.6 SCHEME OF THE CONE OF LIGHT BETWEEN SECONDARY AND FOCUS in the coma free device

y_2s > y_2m

It is	and
It must be	a)
therefore	b)

Being the convergence U_m of the meridional rays in F′_m and the convergence U_s of the saggital rays in F′_s, we get c) since and we can write formula c) d) Formula d) soluted for a_1m will then give our formula 18) q.e.d.

In fig.6 is shown that, if the corrector is placed in the plane X, ( y_3m = y_3s ), the final convergence of the meridional rays U_1m equals the final convergance U_s of the saggital rays, i.e., F_m and F_s are equalized and the astigmatical difference of focal- lengths is eliminated.

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