86 
NOTES AND MEMORANDA. 
tVj tVj Aj caused resolutions on the Nobert 19th band-plate, which 
without it at the same time and with the same illumination could not 
be seen with the same objectives ; with that increased resolution 
the loss of light was not appreciable. Its value was made quite 
apparent by two photographs showing the resolution of AmpMpleura 
pellucida, one by a and the other by a objective, the latter with 
the amplifier. The resolution into transverse striaB in both photo- 
graphs and their appearance is so similar, that were they not numbered 
I could not tell which was obtained by the gV objective.” 
Penetration. — The following note from ‘ Le Microscope,’ by Dr. J. 
Pelletan, p. 65, states a fact that is generally unknown, but which 
ought to be known to everyone who works with the microscope ; 
“ From a purely theoretical point of view an objective with penetra- 
tion is in reality a defective objective.” My experience is that there 
is not one in a thousand of the users of the instrument — especially 
the histologists — but have the idea, and their opinions are governed 
by that idea, that an objective that will show at one sight (or 
focussing) all the strata or planes in a section of a tissue or of 
an inch thick, or, in other words, one that has great penetrating 
power (better defined by the optician’s term, “ depth of focus ”), must 
be the best and most important objective that can be made. Whereas 
it can be demonstrated that such depth of focus is optically incom- 
patible with the best definition, which is the quality above all others 
desirable in a lens. 
Depth of focus is a quality desirable in certain cases and for 
some purposes ; the objectives for such use should be made ex- 
pressly for that property, and the purchasers of such ought not to 
expect them to be capable of the higher histological work of modern 
microscopy. 
There is, of course, a wide difference in definition of objectives that 
possess depth of focus. On the other hand, all objectives that are 
without that quality do not have the maximum defining power. 
The Genus Ligula. — M. Donnadieu holds that all the species 
described by authors as forming this genus are only the different 
phases of development of the same species, or the same parasite found 
in different animals, the so-called genus being simply a species of the 
genus Dibothrium — the Dibothrium ligula. 
Nutriment of Bacteria. — MM. Dupont and Hoogewerff, of Eotter- • 
dam, have investigated the chemical constitution of the materials t 
that nourish bacteria. Test tubes, like those used by Cohn, were 
filled with 20 c.c.m. of the nutrient fiuid, and two drops of bacterium i 
fluid, made with decomposing beans or peas, were added ; the tubes • 
having been first deprived of any atmospheric dust by hot water. 
Care was taken that the distilled water employed contained no 
organisms. Following Mayer, they prepared a normal nutrient fluid 
with 1 per cent, acetate of ammonia, 0 • 5 per cent, phosphate of potash, 
0‘5 per cent, sulphate of magnesia, and 0*05 2 )er cent, phosphate of 
lime. As Mayer stated, they found the most ini 2 )ortant ingredient to 
* Carl Eeddots, in ‘American Journal of Microscojiy.’ 
