could be convenieDtly effecSted, there would be no further dif- 
ficulty. Now to this end I examin d, what may be done not 
ooiy by Chffis alo?ie, but more efpecially by a Complication of 
divers liicceffive Mediums^ as by two or more Glanes or Cry* 
ftals with Water or fome other fluid between them ; all which 
together may perform the office of i?;25 G/«//j efpecially of tha 
Objedi-glafs^ on whofe conftrucStion the perfedion of the in- 
ftrument chiefly depends* But what the refuks in Theory or 
by Tryals have been^ I may poffibly find a more proper occa« 
fion to declare. 
To the Affertion, that Rays are lefs true resetted to a jpoint 
by a Concdve^ than refracted by a Convex, I cannot aflent 3 not 
do lunderftandj thztxht focus of the latter is lefs a line jji/n 
that of the former* The truth of the contrary you wiMh rather 
perceive by this following Table, computed for fuch a Eefiec^ 
tingCencave.^ and Refracting convex^ on fuppofition that they 
have equal Apertures^ and colle£t parallel rays at an equal di- 
ftance from their vertex^ which difl:ance being divided into 
15000 partSj the Diameter of the Concave Sphere v/ill be 
6cooo of thofe parts, and of the Convex, 10000 5 fuppofing 
the sine/ of Incidence and RefracStion to be, in round num- 
bersj as 2 to 3^ And this Table fcews^ how much the exterior 
raySj at feveral Apertures, fall fliort of their principal fs^^e^r. 
Th^ fiirts of the Axis imereepted 
The Diameter between the vcrtiV/L dnd the r ays, \ 
o f the Aperture, Refle^ed, 
2000 
4000 
6000 
8000 
10000 
I 499 If 
14966 
14924 
14865 
14787 
Re framed* 
14865" 
14449 
13699 
1247s 
9472 
The Error hy 
Reflexlofi. j RefraBionl 
^ 
76 
213 
135 
55^ 
I3GI 
5528 
By this you may perceive-, that the Errors of the Refracting 
cQnvex are io farfrom being lefs^ that they are more than fix* 
teen times greater than the like errors of ihQji^flectingConcave^ 
efpecially in great Apertures 5 and that without refped to the 
Heterogeneous eonftitution of light. So that^ however the 
contrary fuppoficion might make the Author of thefe Animad^ 
verfionsrejediS^j?^^'^?(?;2/ as ufelefs for the promoting of Op-^ 
Kkkkk2 tiqii^ 
