ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
217 
MICROSCOPY. 
A. Instruments, Accessories, etc. 
Special Oil-immersion Objectives for Dark-field Microscopy. — 
Simon H. Gage (Science, N.S., 1921, 54, 567-9). The author reviews 
the pioneer work of Joseph Jackson Lister (1880), the Rev. J. B. Reade 
(1887), F. H. Wenham (1850-56), and Dr. James Edmunds (1877) in 
dark-field microscopy. This work was published in Transactions of the 
Royal Society, 1830 ; Trans. Micr. Soc. of London, and Quart. Jour. 
Micr. Science, 1850-56 ; Jour. Quek. Micr. Club, and Month. Micr. 
Jour., 1877 ; Quekett’s “Treatise on the Microscope,” 1848-55; and 
Carpenter’s “ The Microscope and its Revelations,” 1856. 
He points out the two essentials for dark-ground microscopy : — (1) A 
very brilliant light is reeded. Full sunlight was recommended and 
remains the most satisfactory light, although the newly-devised electric 
lights like the small arc lamp and the low-voltage head -light lamps serve 
very well. (2) The large aperture of high-power objectives, especially 
those of the immersion type, which are a necessity with the most 
exacting work, must be overcome, because dark-field condensers cannot 
be constructed with high enough aperture to give a dark-field with these 
high-power objectives. In this connexion two courses are open : (A) to 
so construct high-power objectives that the aperture was low enough 
to give dark- field effects with the dark-field condensers practicable to 
construct ; or (&) to introduce into the high apertured objectives a 
diaphragm to cut down the aperture. 
The second course was that usually adopted, and reducing diaphragms 
of all kinds, with apertures varying from 0*40 to 0 • 90 N.A., have been 
met with, but in many cases they were so constructed that they were 
liable to get out of place, get out of the optic axis, and prove generally 
unsatisfactory. 
After a full examination of the different dark-field condensers made 
it seemed that the best all-round objective to use with them would be 
about 0 • 80 N.A. Such an aperture would give a good dark-field with all 
the standard dark-field condensers, and this aperture is great enough to 
give good resolution on the one hand and the needed brilliancy on the other. 
He therefore appealed to the American manufacturers of microscopic 
objectives to design and manufacture oil-immersion objectives of this 
aperture (0 * 80 N.A.). During the past summer and autumn the Bausch 
and Lomb Optical Company, of Rochester, N.Y., undertook the manu- 
facture of the desired medium-apertured oil-immersion objectives. The 
outcome is all that could be asked, and these objectives are now available. 
The Spencer Lens Company, of Buffalo, also manufacture a 1 • 8 mm. 
oil-immersion of O’ 85 N.A. for dark-field work. Both firms supply 
these special objectives at the same price as their regular oil-immersion 
objectives. 
It may be stated in passing, with reference to these new objectives, 
that they have certain advantages for ordinary bright-field "work. As 
ordinarily employed the oil-immersion objective of high aperture (1 • 40 
