16 THE MICROSCOPICAL NEWS. 
over twenty or twenty-five years ago and found no results to bio- 
logical work,” that the English worked with similar wide aperturés 
twenty-five years ago; we would like to know who made these 
lenses ; and if any of them have been measured by the apertometer. 
We shall be glad to give publicity to any replies to these queries. 
Dr. Carpenter very rightly called attention to the subject of 
penetration: some American and French publications have stated 
that any amount of penetration in a lens is a fault; but Professor 
Abbé has shown us that, in every lens, this attribute is a measurable 
quantity, and an objective is certainly faulty if it does not yield 
this quantity. He has, moreover, shown us that there is four times 
as much penetration in a one inch 40° as In a quarter of an inch of 
40°, and this should influence us in our manipulations. 
One of the absurdities which Dr. Carpenter has endeavoured to 
spirit away is, that a low power lens of wide aperture, say a ;4,ths 
of 150° may be made to do everything. We have pointed out— 
we hope clearly—in our “ Plea for wide Apertures,” that an excess 
of aperture is often given to lenses, with a sacrifice of working dis- 
tance and consequent penetration, qualities not to be overlooked 
in the selection of objectives. 
We have already mentioned (Northern Microscopist, II., 235) 
the case of an inch objective of an angle too wide in our opinion, 
and a still wider aperture has been declaimed against (and very 
properly too) in the following words by Dr. Carpenter :—‘‘I hear 
of Americans making one-inch objectives up to great angles for 
which the Society screw is too small. ‘This makes a very bad 4% 
and spoils it for a one inch.” With certain lenses we cannot en- 
dorse the opinion that it makes a very bad quarter; but we do say 
it spoils the objective for use as a one-inch lens. In fact the first 
portion of the proposition is easily refuted by Dr. Carpenter’s 
remarks upon objectives in his work on the Microscope. On page 
200 (Sixth edition) he says:—‘ A good ;4,ths should resolve the 
larger scales of the Podura without difficulty, and a good quarter 
or fifth should bring out the markings on the smaller scales.” 
Now, according to the Journal of the Royal Microscopical 
Society, working with the A eyepiece, the 54,ths amplifies 125 
diameters, the quarter 200, and the fifth 250.diameters, and prac- 
tical microscopists will admit that these magnifications are none 
too much to enable the various markings to be seen with any 
degree of comfort. In his American address Dr. Carpenter is 
reported to have said, ‘‘ 4 good two-inch should resolve the Podura 
scale, with a sufficient magnification from the eyepiece” ; that is to say, 
the eyepieces must magnify up to 125 diameters for the large scales 
and to 200 for the smaller, or, using the two-inch objective the 
Journal of the R. M.S. tells us, we must use Ross’ E and F eye- 
pieces. 
