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a wide angle to resolve it, viz. the intercostal spicules on Triceratium 
favus in the well-known preparation “ Mud Cuxhaven ” of Moller. 
These spicules being at the angles of the large hexagonal 
structure are comparatively distant from each other, and therefore do 
not need great resolving power ; but by choosing a part where they 
were on the curve of the valve, minute differences in focal depth could 
be instantly detected. There was a certain spicule, which, when 
viewed with a power of 2000 diams. and an aperture of 1'4 N.A., 
appeared dim, while the next one to it was sharp ; this appearance 
was maintained both visually and photographically with various 
powers and apertures down to 250 diams., with an aperture of • 65 N.A. 
With this last power the penetration with an object in balsam is 
nearly equal to 1/10 of one division of Powell and Lealand’s fine- 
adjustment (say '00004 in.), a quantity almost imperceptible. We 
may say, therefore, that with powers ranging from 250 diams. 
obtained with a 1/4-in. objective of '95 N.A. and upwards, there is 
no penetration for normal sight. 
With regard to accommodation depth, it should be especially 
borne in mind that as the accommodation distance lengthens, so also 
does the focus of the lens ; in other w ords as p increases, p also 
increases, because we are dealing with virtual and not screen images. 
A microscopist, when focusing, is said to accommodate at his nearest 
distance : if that is the case, any alteration in focal depth or penetration 
would be in a downward direction, as it is obviously impossible for 
him to accommodate up nearer than his nearest distance of vision. 
So far we have been dealing with the examination of two 
objects at different levels, but when a plane object such as a balsam- 
mounted microphotograph is examined under a low power, the result 
is somewhat different. We shall find at first a sharp definite focus 
confined within very narrow limits. Let this image be placed a 
little out of focus, then by looking steadily at it and by straining the 
eye, it can be seen in what might appropriately be termed an un- 
comfortable focus. Let this process be repeated several times until, 
little by little, we reach a point from which, by no amount of straining 
can we recover the focus. 
The difference between the upper and lower foci obtained in 
the above manner, with a 4-in. objective of *08 N.A. eye-pieced to 
give 10 diams., is 3 mm., and with a 2-in. of • 13 N.A. eye-pieced to 
give 20 diams., it falls to 1*16 mm. We may say, therefore, that 
the penetration with a strained accommodation is about six times as 
great as ordinary accommodation. 
Now, 1 conclude that no one with a focusing adjustment at hand 
would for a single moment think of straining his accommodating 
powers, for if an object at a different level from another object that 
was in the precise focus of the instrument did not come into focus 
perfectly naturally, the observer would instantly have recourse to 
the focusing adjustment, and by that means obtain a proper view of 
