ZOOLOCxY AND BOTANY, MICROSCOPY, ETC. 
91 
Mr. Rousselet also specially recommends tliese small window aquaria 
to those not already acquainted with them, as affording the very best 
means of examining pond water for microscopic life.* 
In Mr. Rousselet’s original figure the lens-carrier was clamped on 
the vertical side of the tank ; we have ventured to show the apparatus 
clamped on the top of the tank, where we think it will be found safer in 
practice. 
(2) Eye-pieces and Objectives. 
New Objective of 1*63 N.A. — This objective is further described 
by Dr. H. Van Heiirckf (see also the paper by Dr. S. Czapski, supra, 
p. 11), who says that- “its advantages have suiq^assed all that could be 
hoped for.” 
The design of the objective was started by Prof. Abbe four years 
ago, but it was only in August last that he was able to complete the 
preliminary calculations and to commence the actual execution of the 
objective, which was finished on the 17th September. 
The objective is 1/10 in., apochromatic, and with an aperture of 
1-63 N.A. A special compensating eye-piece 12 removes the last 
traces of colour. The cover-glasses of Dr. Van Heurck’s objects 
have a refractive index of 1*72, and the slide is approximately of the 
same index ; both are of flint glass. The diatoms are melted in the 
cover-glass. The mounting medium has an index of 2*4, and the 
immersion fluid (monobromide of naphthaline) an index of 1 • 65. The 
aperture of the immersion condenser is 1*60 N. A., and the upper lens 
is of flint, for utilizing the most oblique light. Monobromide of 
naphthaline is used here also as the immersion fluid. 
The lenses of the objective are thus disposed : — 
(1) Front lens (more than a hemisphere) of flint. Index 1*72. 
(2) Achromatic lens. 
(3) Crown-glass lens. 
(4) Achromatic lens. 
(6) Correcting lens (three glasses). 
Three of the lenses are in fluorite. 
Prof. Abbe considers that the difference between the indices of 
the cover and the immersion liquid notably favours the resolution. 
In regard to its management. Dr. Van Heurck has used it daily for 
two months, and for hours together, and he finds it in every way as 
practical as other objectives of large aperture. 
“ In oblique light Ampliipleura is entirely resolved in beads, as 
clearly as we see Pleurosigma with the best existing objectives, the 
beads being much closer than the previous imperfect resolutions had led 
one to believe. Repeated measurements of photographs show that it 
has 3600 transverse and 5000 longitudinal striae per millimetre. It is 
not, therefore, surprising that there has hitherto been so much difficulty 
in showing these beads. 
It is only for these beads that oblique light is required. All the 
other difficult tests, such as Frustulia saxonica, Surirella gemma, and 
* Journ. Quek. Micr. Club, iv. (1890) pp. 53-4 (1 fig.). 
t Cf. Bull. Soc. Belg. Micr., xv. (1889) pp. 69-71 ; Jouru. de Microgr., xiii. 
(1889) pp. 527-8. 
