On a Modification of the " Slit " for Testing Angle. 23 
I may as well state here what is of real importance in using 
such an objective as the |th, i. e. of maximum (or large), angle and 
long working distance through cover. That objective goes best 
with the thicJcer covers, therefore the thin covers ^ to should 
be supplemented with glycerine instead of water. This gives best 
command of all thicknesses of cover, notably if the objective is 
corrected for lest work through the nearly thickest covers it will 
penetrate. 
Boston, Nov. 25, 1874. 
