Note.-The wave length assumed in the above Table is, as stated, 0·5269 μ, but it is not unimportant to observe, that if the

=

point taken had been, as nearly as may be, midway between E and F, i. e. 0.508 μ, the numerical aperture would itself have been the true measure of resolving power, being exactly equal to the number of hundred-thousands of lines in an inch; thus giving 100,000 lines as the maximum of a dry lens, 133,000 as that of a water-immersion, and 152,000 as the ideal maximum of a homogeneous-immersion objective with crown-glass covers.

μ

As the resolution by the same objective is affected by the intensity of the illuminating beam, by the colour of the object, and other circumstances, it is not possible to assign an absolute measure -using the line D (=0·5889 μ), for instance, in the computation, the resolving power would be found to be 10 PER CENT. less than the tabular results: on the contrary, if the line F (=0·4861 μ) were selected, the resolving power would be 8.4 PER CENT. greater, or taking the line = 0.40 (near H1), as probably sufficient for photographic resolution, the maximum number of lines which could be delineated by a dry lens would be 127,000, by a water immersion 163,910, or, by a homogeneous immersion objective (under present conditions) 193,040, although, if flint-glass covers with suitable homogeneous fluid were adopted, the ultimate limit might, on this hypothesis, be extended to, say, 200,000; and on the assumption that the space between each of these lines was equal to the lines themselves, it follows that the width of each line would be 400.000 of an inch, but it by no means hence follows that this would be the ultimate photographic limit for a single line.

There is no loss of aperture on objects mounted in balsam, or any more highly refractive medium (as was formerly supposed by many, myself included), under either of the systems, in fact the full resolving power (with transmitted light) can be attained only if the object is mounted in a medium whose refractive index is equal to, or greater than, the numerical aperture of the objective, so that if ever homogeneous objectives are constructed for flint-glass covers, some more highly refractive medium than balsam must be used for mounting the objects, such as bisulphide of carbon or oil of cassia.

XLII.-Aperture Measurements of Immersion Objectives expressed as "Numerical Aperture."

By JOHN MAYALL, jun., F.R.M.S.

(Read 12th November, 1879.)

As it is probable that apertures will be expressed, in future scientific discussions, by the nomenclature suggested by Professor E. Abbe, i. e. "Numerical Aperture," the following list of actual measurements may be interesting to microscopists:

Zeiss's homogeneous immersion (Abbe's formula,)
1879)

Tolles's oil immersion (1879)

"Numerical Aperture."

1.40

+++

1.30 +

1.30

Tolles's water'immersion (1875)

Powell and Lealand's oil immersion (1879)

1.20

(1876)

1.20

1.20

water

1.15 +

formula (1876) ..

Prazmowski's

Zeiss's

I am indebted to Professor R. Keith (of U.S.A.) for this measurement. He measured the balsam angle to be 115° (the highest aperture attained at that date), which would be about 1.28 of "numerical aperture." The other measurements were made by myself, and several were verified by Professor E. Abbe.

The above list does not include those immersion objectives, by various makers, which yielded measurements of balsam angle notably less than 82° (= 1·00 num. ap.). The signs + indicate in which direction the measurement tended.

RECORD

OF CURRENT RESEARCHES RELATING TO

INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, &c.,

including Embryology and Histology generally.

ZOOLOGY.

A. GENERAL, including Embryology and Histology
of the Vertebrata.

Gestation of the Armadillo.*-Mr. Milne-Edwards refers to the presence of four foetal (nine-banded) armadilloes in a common chorion; the phenomenon may, he thinks, be explained either (1) by a number of ovules having been enclosed in the Graafian follicle, inasmuch as these ovules might also be enclosed in a common granular layer, and this layer would, on accompanying them into the oviduct and thence into the uterus, be converted into a common chorion; or (2) the effect may have been produced by the disappearance of the granular layer of each ovule and by the subsequent investment of the four fecundated ovules by a layer formed by the walls of the oviduct or uterus;-but this could not happen unless the secondary (amniotic) chorion had been absorbed, or had never been formed; or, (3) the four amniotic chorions might have fused at their points of contact and have undergone absorption at all but their peripheral portions.

Vitality of the Spermatozoa of the Trout.t-The details of M. Henneguy's experiments will be seen by the following table, which shows the number of ova that became developed after they (having been impregnated), were subjected to the following agents:

The fishes produced exhibited no difference to those developed from ova which had been fecundated in the ordinary manner; and it is concluded that doses of alcohol, &c., which are sufficient to kill Infusoria for instance, have no effect on the spermatozoa.

Experiments on Development.‡-MM. Pouchet and Beauregard describe some experiments in which they removed from eggs a small quantity of their liquid albumen and replaced it by a certain quantity (half a gramme) of sugar; and then reclosed the eggs in the manner they have already described. Experiments were made on eighty eggs, and of these twenty-eight were opened during the first thirteen *Comptes Rendus,' lxxxviii. (1879) No. 9.

'CR. Soc. Biol.' for 1877 (1879), p. 274.

Ibid., p. 338.