ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
93 
lenses ; they adhered to the usual form of two doubles with a single 
front. Among the first and most successful was Swift ; an early example 
is on the table. This lens is a 1/5, and was sold for half the price 
of the English 1/4 of that day. 
A new competitor appears on the field, viz. Reichert of Vienna. 
The example before you is one of the first batch of his lenses that came 
to this country; it is a 1/7 of 0-84 N.A., very well corrected and very 
well finished. Its price was 2Z., and at that time there was nothing- 
made here that would at all compare with this lens for three times that 
sum. Next in order comes Leitz of Berlin, who was mainly known 
by his 5Z. oil-immersion, and now as I have brought uj3 the history to 
recent times I will give no more particulars with regard to achromatics, 
but go straight to semi-apochromatics. 
I call these lenses “semi-apochromatics” because, while they are 
not “ apochromatic,” they possess a higher degree of achromatism, due 
to the employment of Jena glass in their construction, than previously 
i)ossible with the old glass. Among others the most remarkable 
instance of the capabilities of this Jena glass will be seen in the pro- 
duction by Leitz of two lenses, a 2/3 of N.A. 0-26 and a 1/8 of 0-88 
N.A. The 2/3 is a remarkably fine lens which has no achromatic rival, 
even though it consists of only two pairs, and the 1/8, which to my 
fancy is rather too high in power for its aperture, by far surpasses any 
similar achromatic lens. Now when we remember that the price of 
these two lenses together is only the modest sum of 2Z. 8s., we are in 
a position to realize the great strides the manufacture of Microscope 
lenses has made quite recently. I hope to show a blow-fly’s tongue 
under one of these presently. 
In this country Swift has made use of Jena glass in the production 
of “Students’” lenses with great success; some dry 1/6 and 1/8 
may be specially noted as having eclipsed every similar lens, and 
this without entailing any extra complication in construction. 
Last week Mr. Baker sent me a new Reichert 51. oil-inunersion 1/15 
of 1*25 N.A., which on measurement I found to be a true 1/12 of 1*24 
N.A. Under this lens I am exhibiting the secondaries of Coscinodiscus 
Asteromphalus. 
This lens is the finest oil-immersion I have ever seen, excepting 
only the apochromatics. The spherical aberration is beautifully 
balanced, as can be seen by the large cone of illumination used, viz. 
the full aperture of the Zeiss achromatic condenser, with bull’s-eye. 
Beyond this, however, the lens falls off. I know of no similar lens 
that will stand such a severe test. The object I have chosen has thick 
intercostal silex, and therefore is especially one to show up any colour 
left outstanding. The thicker the silex the stronger the colour 
(hence an excellent means of determining roughly the thickness of 
diatomic structures). Most lenses show this same object deeply 
coloured. With another object such as a Navicula Bhomboides (Cherry- 
field) in balsam, the silex on either side of the raphe will appear as 
very pale lilac. The lens also shows admirably a diflSicult test such as 
the secondaries on Aulacodiscus Sturtii. Such a lens cannot fail to 
play an important part in the microsco2)y of medical and science 
schools. 
