ey ee | Royal Microscopical Society. 307 
variety of eye-pieces, I may be allowed to recommend the follow- 
ing for the }th and #,th with searcher :— 
Focal length. 
Field glass crossed lens eee finches: 
Eye lens crossed lens .. .. «. «. 1} inch. 
The aberration of this eye-piece is less than that of many in 
use. 
For parallel rays it becomes achromatic when the interval be- 
tween the lenses is two-and-three-quarters of an inch.* 
With Powell and Lealand’s new 1th, a long draw-tube, searcher 
and C eye-piece, an extraordinary bright amplification of the For- 
mosum, est?emated at 4000 diameters ; but this was not measured, 
as the objective was taken away by the makers, having been lent 
for the occasion. 
The favourite power with the }-inch has been 700; the pene- 
tration or depth of focus appeared greatly increased, and many 
objects were more easily defined with this kind of treatment, espe- 
cially where great thickness of glass cover was required to be seen 
through. 
Postscript.—The writer regrets that he is unable to introduce 
sufficient diagrams to illustrate all the points of this imperfect 
paper. 
* Half the sum of their focal lengths. 
VOL. IV. Z 
