ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
659 
MICROSCOPY. 
a. Instruments, Accessories, &c.* 
Cl) Stands. 
Altmann, R.— Ueber die Verbesserungsfahigkeit der Mikroskope. (On the 
possibilities of improvements in the Microscope.) Part II. 
Arch. f. Anat. u. Physiol ., Anal. Ahtheil. , 1889, H. 5, 6, p. 326. 
Anderson, R. J.— A panoramic arrangement for the Microscope. 
Internat. Monatsschr. f. Anat. u. Physiol ., VI. (1889) H 8, p. 289. 
Bernard, P. — Note sur un Microscope compose du 18 e siecle. (Note on a 
compound Microscope of the 18th century.) 
Journ. des Sci. Med. de Lille , XI. (1889). p. 1. 
Heitzmann, n Die Zukunft der Mikroskopie. (The future of Microscopy.) 
Wiener Med. PL, XII. (1889) Nos. 37, 39. 
Olivier, L. — Histoire des Microscopes. (History of Microscopes.) 
La Nature , XVII. (1889) pp. 267, 314. 
Poli, A.— Le Microscope et sa theorie. (The Microscope and its theory.) 
Rev. de Pot., VII. p. 20. 
(2) Eye-pieces and Objectives. 
The Achromatic Object-glass. — J. Godfrey writes:! — “I am 
glad to find our correspondent 4 Prismatique ’ coming to the front 
again, and as an amateur optician I write to ask his opinion upon 
a curious point. The glass I have always worked upon has been 
Chance’s hard crown and dense flint, and after all sorts of experi- 
ments with different combinations of curves, I have found that 
a very good combination is to make the crown lens equiconvex, and the 
curves of the flint in the proportion of ten to one double concave. Of 
course I am well aware that these curves are foundation curves only, and 
that delicate and final corrections are indispensable ; the workman, so I 
find, can only select curves to work up to, and alter, according to his 
experience and manual skill. Now I find — and this is the result, not of 
theory, but of experience — that with these proportions of the curves the 
flint lens corrects the achromatism of the crown slightly more at the 
marginal zone than it does at the centre. For example, if the flat lens 
so far over-corrects the crown as to eliminate the irrationality of the 
crown lens with respect to the red of the spectrum for the outside zone, 
there then remains, as I have found by practice, a minute residuum of 
the secondary spectrum in favour of the crown lens in the centre of the 
object-glass, a faint trace of red, which of course is not obtrusive, but it 
is there. Now I want to eliminate this want of balance between the 
outside and centre of the object-glass. At present I am very busy working 
upon two very fine and massive discs of Chance’s hard crown and dense 
flint, and the object-glass will be 7 in. clear aperture ; of course this is 
not the first glass I have made. My present 5 in. will show a curiously 
mottled and indented terminator upon Venus, and she is a terrible planet 
to define. Now I want 4 Prismatique’s ’ opinion upon this point. I propose 
to make my crown-lens equiconvex, 27 • 5 in. radius, and the focal length 
of the flint to be 49 # 6 in. This will give the proportions of the focal 
* This subdivision contains (1) Stands; (2) Eye-pieces and Objectives; (3) Illu- 
minating and other Apparatus ; (4) Photomicrography ; (5) Microscopical Optics 
and Manipulation ; (6) Miscellaneous, 
f Engl. Mech., li. (1890) p. 118. 
