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II. — On Angle of Ajoerture in Belation to Surface Markings 
and Accurate Vision. 
By Henry James Slack, F.G.S., Sec. E.M.S. 
(^Read before the Royal Microscopical Society, May 5, 1875.) 
The fascinating character of diatoms, and the desire to resolve 
their surface markings, have exercised a most important influence in 
this country, and on the Continent, upon the labours of opticians 
in the production of objectives for the microscope. 
It was soon found that glasses upon the old patterns with 
small angles of aperture could scarcely show any objects of this 
description, while others, even when inferior in their corrections, if 
of much larger angles achieved considerable success. This stimu- 
lated the manufacture of large-angled glasses, and microscopists 
vied with each other in obtaining from the most enterprising 
makers objectives with angles so out of all reasonable proportion to 
their magnifying powers and focal distance from the object, that 
they were nearly or quite useless for general purposes of natural 
history and physiological research. Objectives thus became divided 
into two varieties, which from their persistence may be termed 
species, — one good for surface markings and nothing else, the other 
only capable of displaying the easier of such markings, but service- 
able for general investigations, and contributing to perhaps nine- 
tenths of the useful discoveries made with microscopic help. 
Physiologists did not attempt to conceal the scorn with which 
they looked at the mere displayers of diatom dots, and for the most 
part gave up all hope of obtaining objectives suitable to their 
researches, and at the same time possessing a high degree of that 
resolving power upon which the dot showers depended for their 
success. It was in fact generally supposed that resolving power 
and penetrating power stood in relations of irreconcilable hostility, 
and it was also supposed that a considerable amount of chromatic 
error was essential to the sort of correction best fitted for the dot 
work. No one could deny that up to a certain date the best dot- 
displaying glasses had considerable chromatic errors, and that 
other glasses with better chromatic corrections did not show difficult 
dots so well. Had it been considered that all chromatic aberration 
involves spherical aberration, the belief in any theoretical necessity 
for leaving considerable chromatic error in order to ensure sharp 
definition would scarcely have become so prevalent. It is obvious 
that the best image of an object would be formed by bringing all 
the hght rays from it into their right places, and as chromatic 
errors bring some into wrong places, they must cause spherical 
distortion. 
