The President's Address. 
69 
parable, either in tlie convenience of its meclianical, or tlie per- 
fection of its optical arrangements, with the instruments of our 
best makers. Unquestionably the best foreign objectives were 
those exhibited by M. Hartnack (the successor, I believe, 
of Oberhaiiser), in the French department. Some of these 
were very deep powers ; the deepest of these being only 1 m. m. 
focus, and therefore about equal in power to the Vyth of 
Messrs. Powell and Lealand ; but an evening was devoted to 
the comparison of these lenses at my residence, and the 
superiority of the latter (which must be looked upon as a 
triumph of artistic skill) was freely admitted. 
There is one point of construction, frequently met with 
in the foreign objectives of high power, to which I would 
especially call the attention of our professional members, 
as, from what I have seen, I do not think that it has 
received in this country the attention it deserves ; it is their 
being corrected for immersion in water, that is, that a plate 
of water should interA^ene between the objective and the 
covering-glass of the object. From the increased facility of 
transmission of the oblique rays through a plate of water, 
the quantity of light under any given condition of illumina- 
tion is obviously increased. With a -j-oth objective of moderate 
angular aperture, which is corrected for immersion in water, 
I have, I think, in some instances, obtained better definition 
than by any other means. 
In regard to the angle of aperture of objectives, it may be 
remarked that, for physiological purposes, and, indeed, for 
almost all practical purposes except revealing the markings 
of diatoms, large angle of aperture is not necessary, nor 
even desirable, being incompatible with that far more 
generally useful quality of a good objective, penetrating 
poiver, which is, in fact, synonymous with depth of focus, 
that is, the extreme distance of two planes, the points of 
which are, at the same time, sufficiently in focus for the 
purpose of distinct vision. This distance will obviously 
increase as the angle of aperture diminishes, just as in a 
landscape camera the fore- and back-grounds can be brought 
into sensible focus simultaneously only by the employment 
of a diaphragm with a small aperture, which greatly dimi- 
nishes the angular aperture of the incident pencils. But, 
at the same time, it must be borne in mind that illumina- 
tion, cceteris paribus, increases or diminishes with the angle 
of aperture, and the most efficient objective will be that in 
which the best compromise is effected between these two 
conflicting requisites. 
I may here mention a very satisfactory method of deter- 
