64 
Transactions of the Roijal Microscopical Society. 
means dazzling, the frustule undistorted, and the striae clean and 
black on the white ground, very little colour-aberration being per- 
ceived. With other objectives there was more or less colour- 
aberration and distortion, both which faults were in one or two 
cases very conspicuous, although in the part of the frustule most 
sharply focussed upon the striae were handsomely brought out. 
The objectives with which I thus succeeded, ranged all the way 
from a £tli to -rgth immersion. I will add that the objectives 
which resolved Amphipleura pellucida under these trying circum- 
stances, when used in the ordinary way with this or other test- 
objects, displayed an exquisite perfection of definition which it 
would be hopeless to expect to attain with objectives of less angular 
aperture. 
As it is no part of my purpose in this communication to provoke 
ill-tempered discussion of the merits of individual makers, I will 
not append a list of the results obtained with the various immersion 
objectives I have tried in this way. The apparatus can be con- 
structed for a few shillings, and those who take the trouble to use it 
will soon see to which of the three classes any particular objective 
they may test belongs. 
