172 THE MICROSCOPICAL NEWS. 
images such an objective would present in the much more unfavour- 
able case of the ordinary observing position. Nor can the result 
be considered as sufficiently characteristic even of the “ resolving 
power ” in its more general attributes. 
Nor can the test of “resolving power” by direct light be 
estimated at a much higher value. In the neighbourhood of the 
limit of resolution corresponding to this form of illumination, all 
direct light passes through the central zone, and all diffracted light 
through the peripheral zone of the aperture. 
From the point of view presented by the theory here propounded, 
another method offers itself, which, while employing the usual tests, 
brings directly into light the particular ° points which mainly in- 
fluence the quality of performance during ordinary use of the 
microscope. If it be desired to test, in a most critical way, the 
conditions of exact co-operation of ‘pencils of light which pass 
through every part of the aperture, there are truly no better means 
than those afforded by natural objects of the diatom class and 
insect scales, provided that the mere fact of accomplished 
“resolution” is not made the chief consideration, but that the 
exact constitution of the total image produced by the objective is 
studied. 
The considerations adduced lead to certain rules respecting the 
right proportion between focal distance and angular aperture, which 
are opposed in many points to the hitherto prevalent practice. 
Since theory demands a limitation of angular aperture of 110° 
for all dry combinations, the calculation of minutest detail accessible 
to such objective is readily made; and it may be shown that if 
“resolving ” power be not unfairly exalted at the cost of the general 
excellence of the lens, there can be no question of detail which a 
practised eye would not recognize with a good amplification of from 
4 to 500. Now according to the present standard of technical 
constructive means, such an amplification may be gained with an ° 
objective of 3 mm. (¥% English inch), even if the attribute good be 
interpreted a little more strictly than is often done. With immersion 
lenses, the physical limit of “ resolution,’ even where the angular 
aperture is the highest attainable, does not extend so far that an 
amplification of from 7 to 800 will not be fully equal to it; and 
this amiplineauon would be gained with ease with a well-constructed 
objective of 4, inch focal length. It may be admitted that an 
amplification exceeding the minimum here given as theoretically 
necessary, might greatly facilitate observation and render it more 
certain 2f the additional amplification be as correct as can be 
possibly made, although it would not occasion any new facts to be 
seen. Yet one can scarcely estimate the significance of this empty 
amplification far beyond the limits stated, and I therefore come to 
the conclusion that the scientific value of an objective whose focal 
