NOTES AND MEMORANDA. 
153 
longitudinal direction, are smaller than those between the transverse 
rows, while in N. crassinervis the opposite is observed, and the trans- 
verse intervals are much narrower than the longitudinal ones.” 
Keferring to the use of monochromatic light by Amici, he stated 
that he used “ a prism of 35° of flint glass, in which the solar ray is 
decomposed in immersion and emersion,” and that a Paris optician 
was going to make a cheap and suitable heliostat. He usually 
employed the blue, or green, or extreme violet rays, but practice was 
necessary with the latter to get used to the demi-obscurity.* 
The New Oil-immersion Object-glass. — The Rev. W. H. Dallinger 
writes to ‘ Nature ’ : — “ As a piece of workmanship this lens is extremely 
fine ; and it can be used with quite as much ease as an ordinary im- 
mersion L-inch objective. It works admirably with Powell and 
Lealand’s ordinary sub-stage condenser, with Wenham’s reflex illu- 
minator, and with the small plano-convex lens which the maker sends 
with it to be fastened to the under surface of the slide with the oil of 
cedar wood. But I have also secured admirable results with the 
illuminating lens of Powell and Lealand’s supplementary stage, 
which gives entire command over the angle of the illuminating ray. 
“ The spherical aberration is beautifully corrected, the field being 
perfectly flat. The colour corrections are, so far as the lens goes, 
equally perfect ; but are somewhat conditioned by the dispersive 
power of the oil, which can be modified readily. The sharpness and 
brilliance of the definition which this lens yields is absolutely unsur- 
passed, in my experience ; and it has a very great power of penetration. 
“ I tested it with a series of tests with which I have proved and 
compared the glasses of various milkers in England, the Continent, 
and America for some years. Up to the time of receiving this lens, 
the ^-inch that had done the most in my hands, was one of the ‘ new 
formula’ lenses of Powell and Lealand. It is but justice to say that 
all my most crucial tests were equally mastered by the lens of Zeiss. 
I have not been able to do more with it than with the English glass, 
but the same results can be accomplished much more readily. The cor- 
rection has to be brought into operation, and careful adjustment 
made, to get the finest result with the English lens ; but the C-erman 
glass has simply to be brought into focus, and the best result is 
before the observer, provided that the light has been adjusted in the 
most efficient manner. It is true that for sharp and perfect definition 
we must be careful to adjust the length of the draw-tube ; in working 
this lens there is much need of attention to this matter ; and speaking 
from a practical point of view, it takes the place, in securing crisp 
definition, of the screw-collar adjustment, although, of course, much 
easier of application. But it is so easy to work the lens with fine 
results on the more delicate tests, that I think that those who make 
the resolution of these their primary object in the possession of a 
microscope, can scarcely fail in securing their utmost desire. It is a 
glass pre-eminently suited for the resolution of difficult lined or 
beaded objects. 
“ A7uphipleura pellucida is easily resolved into delicate beads when 
the frustules are moderately coarse ; and almost any that can be met 
with are resolvable into lines ; and this when these diatoms are 
* ‘ Bulletin de la yoc. Botanique de France,’ t. xxiv., 1877. 
