554 



NATURE 



lOctober 8, J 89 1 



tpurpose in hand ; but I givj the curves in question oMained 

 experimentally for seven lenses of difffrent types in my 





Fig. 3. — English Portrait, 





Fig. 4 —English Portrait. 



[Fig. 5 is peculiar in having the i;sual rela ive posf.ons of the 

 primary and secondary foci reversed.] 



i\Q. 5.— French Portrait. 



possession (see Figs. 3 to 9). All these lenses except Fig. 5 

 are by makers reputed to be the best. 

 NO. I 145, VOL. 44] 





Fig. 7. — French Wide-angle Ucublet. 



The ordjnates of these curves are the distances of the primary 

 and secondary foci from a plane through the principal focus at 



