32 THE MICROSCOPICAL NEWS. 
is equivalent to the visibility of parallel lines, at ten inches distance 
from the unaided eye, having a separation ranging from 5+ to of 
an inch, and this estimate may form the basis for a calculation of 
the magnifying power required by different observers in order to 
see lines having a separation amounting to zg$o5 Of an inch. We, 
therefore, find that an amplification ranging from 116 to 232 
diameters is necessary to resolve such lines, and that a two-inch 
objective of the very wide angular aperture of 25° will require eye- 
pieces equal in power to Ross’s E or F, the latter being the most 
powerful one made in this country, and that in some cases even 
this will be insufficient to meet the requirements of individual 
observers, and the draw-tube will have to be used in order to obtain 
the large amplification of 232 diameters. Powell and Lealand’s 
highest eye-piece yields only 150 diameters with a two-inch ob- 
jective. Is it not possible that the Doctor may have been charmed 
with the well-defined images produced by some of the wide- 
angled lenses of the transatlantic opticians? But what does ¢heory 
say as to their penetrating power? A first-class two-inch lens in 
this country should have an angle of 14°, and focal depth in the 
monocular instrument (under similar conditions of magnifying 
power, illumination of the object, &c.) is in the inverse ratio of 
the numerical aperture. Therefore, if this quality in the English 
two-inch lens be expressed by 1.0, in the American lens of 25° it 
must be expressed by .56, or little more than one-half. In order 
to resolve 25,000 lines to the inch an angular aperture of 31° will 
be required, and an amplification of from 145 to 290 diameters, 
and these conditions may be obtained in our English $rds ob- 
jective and Ross’s C and D eye-pieces. 
Thus ¢ieory shows that the lowest powers in use in this country 
that can be expected to resolve the larger Podura scales are the 
best one-inch and two-thirds objectives, with amplifications of 116 
to 290 diameters. ‘This will, I think, be found to accord with 
observation. Some of you may think that a better test for a two- 
inch lens for ordinary biological work is the old-fashioned one of 
an opaque injection, or the pollen of the Mallow zz sz#u upon the 
anther. For my own part, I prefer still to attach my faith to the 
“ Revelations,” rather than adhere to their author’s latest dicta as 
to “the best objectives for biological work.” 
