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MICROSCOPIC POWER OF OBJECTIVES. 
ON THE MEASUREMENT OF THE MAGNIFYING 
POWER OF MICROSCOPE OBJECTIVES, 
WITH EXHIBITION OF 1-25th INCH WATER-IMMERSION 
OBJECTIVE OF POWELL AND LEALAND.* 
BY WILLIAM P. MARSHALL, M.I.C.E. 
This objective is of exceptionally high power, having a 
magnifying power of from 1500 to nearly 4000 diameters, 
according to the eyepiece used. The aperture of the objective 
is very small, and the face lens is not larger than a small 
pin’s head, this lens having to be worked to as perfect a 
figure as the larger lenses. This objective requires delicate 
and careful handling, because of the extreme thinness to 
which the metal mounting of the face lens is reduced in 
order to allow of its being brought close enough to the object 
for bringing it into focus ; and, in consequence, even a slight 
touch upon the metal mounting of the face lens would be 
liable to cause serious injury. 
The objective is not, however, difficult to use, and with 
suitable arrangements it is practically capable of being used 
as easily and expeditiously as one of low power; provided 
that the cover-glass of the object is of suitable thinness for 
focussing so high a power. This cover-glass is not required 
to be excessively thin in the present case (‘005 inch is a 
suitable thickness), because the objective is a water-immersion 
lens, and, consequently, works with much greater clearance 
in proportion than a dry objective ; and this l-25th inch 
objective allows nearly as much clearance in working as a 
l-8th dry objective. The special focussing arrangement used 
(the plan of Dr. Anthony) is a thin loose brass plate, laid 
upon the microscope stage under the object to be examined ; 
this plate has a central aperture 7-8ths inch diameter, and 
projects l-8th inch beyond the stage at the right-hand end, 
when it is pushed home to the stop at the left-hand end. 
The projecting top corner of the plate is touched lightly with 
the finger, and slightly lifted off the stage and lowered again 
continuously, during the course of bringing down the 
objective in focussing; and this gives instant warning of the 
approach to the object by momentary glimpses of the object 
being obtained in the lifting of the plate, enabling it to be 
promptly brought into focus. With this arrangement, so 
* Transactions of the Birmingham Natural History and Micro¬ 
scopical Society. Microscopical Section. Read March 1st, 1887. 
