Journal of the Royal Microscopical Society, Containing Its Transactions and Proceedings and a Summary of Current Researches Relating to Zoology and Botany (principally Invertebrata and Cryptogamia), Microscopy, &c. ...

displayed on this diatom, according as we admit or exclude particular sets of spectra. The results obtained from this manipulation may be summarized in three propositions :—

(1) The same structure will give different images when the diffraction-beams are made different.

(2) Different structures will give the same image when the diffraction-beams are made similar in each case.

(3) (the proposition which is most pertinent to our present subject). The microscopic image of a structure is never in perfect accordance with its actual composition, or true structure, unless the whole of the diffraction-pencil is admitted to the Microscope; or, in other words, the image is always more and more dissimilar from the true structure in proportion to the greater number of diffraction-pencils which are excluded from the Microscope.

The diagram will serve to illustrate the practical application of the last proposition to the examination of diatoms. If the structure

FIG. 26.

is coarse,' the diffractionbeams will all be included within a small space around the central pencil (the inner circle of the figure), and in this case an objective, even of limited aperture, will receive them all, and we shall have an image of the true structure. If the object is finer, the limited aperture will not be sufficient to take up all the diffraction-pencils, but a larger aperture (the middle circle of the figure) will. Still more minute structure will require a still larger aperture, as is shown by the outer circle.

Now the elements of S. gemma are of such fineness that they far surpass the limits of any aperture that we are able to obtain at the present day. Aperture is limited by the refractive index of the glass of which the objectives are made, and that of the immersion fluid, cover-glass, and slide, and hitherto we have not been able to obtain more than 1·47 N.A. out of a possible 1.52. An aperture even of 1.52 would take up but a part of the diffraction-beams to which the structure of S. gemma gives rise, and, therefore, with our widest apertures it is impossible for us to see its true structure. I need not give the figures of the calculation here; but the fact is that to see the true structure in reality, we should require objectives, slides, and immersion fluids far surpassing in refractive index any substance hitherto known to exist in nature.

To quote Prof. Abbe: All speculations as to the true structure of even P. angulatum, so far as they depend on microscopic vision, are mere phantoms, castles-in-the-air. No human eye has ever seen,

or will ever see, the complete diffraction-spectra arising from a structure of this minuteness, nor will any Microscope ever show an enlarged copy of it, so long as the spectra cannot be observed in a medium of at least 5.0 refractive index, and by an objective of 5.0 N.A., which, as far as our present knowledge goes, is an impossibility. The Microscopes of the present day admit relatively a small central portion of the whole diffraction-pencil of the valve-i. e. the incident beam and the six spectra of the inner circle. But this portion is also yielded by a multitude of other objects which are endowed with an alternation of superficial or internal molecular structures which cross each other in two different directions at an angle of 60°. Such structures may be formed in various widely different ways; it may be by rows of spherules or other prominences of any shape whatever; rows of internal vacuoles of any figure, or the mere internal alternations of molecular aggregations within a perfectly transparent and smooth silica film. And yet all of these yield with central light the identical circular field of the angulatum valve, even to the most minute particular. But although these spectra are identical as far as the six inner spectral beams are concerned, they may be vastly different in regard to some or all of the more widely diffracted pencils which are not admitted by the objective.'

However expert, therefore, a microscopist may be (and every one knows the high point which Mr. Nelson has reached), he must not delude himself with the notion that perfection in technical dexterity enables him to determine the "true" structure of objects whose real structure cannot be revealed with our present appliances by any amount of manipulation. The greater his own reputation in this respect, the more undesirable it is that he should proclaim such misleading views, to the perplexity of his less experienced brethren."

Resolution of Amphipleura pellucida by Central Light.-This has been the subject of some controversy in America. Mr. A. Y. Moore* considers the real explanation of the resolution when the mirror is central to be that the edge of the front cell of the objective radiates the light and all light reaching the bottom of the slide at a greater incidence than the critical angle is reflected upwards and enters the lens after having passed through the diatom.

Dr. H. J. Detmers † considers this explanation to be quite untenable and the true cause to be that "the resolving rays are reflected from the (externally convex) internally concave surface of the edge of the immersion fluid."

Prof. A. Y. Moore, in reply,‡ insists upon the correctness of his view and the insufficiency of that of Dr. Detmers, inasmuch as the field of view takes the colour of the metal of which the front

cell of the objective is made. This would not occur if the light were reflected from the edge of the drop of immersion fluid.

p. 595.

*The Microscope, iii. (1883) pp. 49-51 (1 fig.). Cf. this Journal, iii. (1883) + Ibid., pp. 197-201.

Ibid., pp. 201-4.

ALBERTOTTI, G., jun.-Sulla Micrometria. (On Micrometry.) [Post.]
Ann. di Ottalmologia, XI. (1882) pp. 29-30 (1 pl.).
Klin. Monatsbl. f. Augenheilkunde, 1882.

ANON.-The Wonders of Optics.

[Inquiry for "a glass that I can see through paper or leather, and if you
have one please to be kind enough to send me the price of it at once";
and reply of editor, "Punch a hole in the paper or leather."]
Micr. Bulletin, I. (1883) p. 7.

BARLOW, T.-See Tolles, R. B.

Bausch and Lomb Optical Co.'s new pattern" Investigator Improved " Microscope, and 1/4 in. objective.

[Coarse adjustment moves nearly 2 in. higher-pillar heavier and higherseparable swinging tail-pieces-Objective with extra large working distance.]

The Microscope, III. (1883) p. 239.

BELL, J. S. B.-Warm Stage and Stage Condenser for Diatomaces. [Warm stage post. Stage condenser "simply an addition of a shutter to the hemispherical lens . . . . similar to that used by Powell and Lealand."]

Micr. News, IV. (1884) pp. 19-20.

BLACKHAM, G. E.-The relation of aperture to amplification in the selection of a series of Microscope Objectives. [Post.]

Proc. Amer. Soc. Micr., 6th Ann. Meeting, pp. 33-41.
Discussion, pp. 227-31.

See also Tolles, R. B.
BRADBURY, W.-The Achromatic Object-glass, XXIX.

Engl. Mech., XXXVIII. (1883) pp. 258–9 (1 fig.).
On Eye-pieces.
(1884) pp. 401-2.
BULLOCH, W. H.-New Congress Nose-piece. Patented 1883. [Supra, p. 118.]
The Microscope, III. (1883) p. 218 (2 figs.).
Also U.S.A. Patent, No. 287904, of 23rd January, 1883.

C., J. A.-See Penny, W. G.
CARR, E.-Microscopic Test Objects. [Supra, p. 138.]

Engl. Mech., XXXVIII. (1883) p. 280. COHEN, E., and GRIMM, J.-Sammlung von Mikrophotographien zur Veranschaulichung der Mikroskopischen Structur von Mineralien und Gesteinen. (Collection of micro-photographs for the demonstration of the microscopical structure of minerals and rocks.) Parts IX. and X. (conclusion). 38 pp. Plates 65-80. 4to, Stuttgart, 1883.

COHN, F.-Bicentenary of Bacteria.

[Calls attention to the fact that, in a letter dated 14th September, 1683, A. van Leeuwenhoek gave notice to the Royal Society that with the aid of his Microscope he had discovered in the white substance adhering to his teeth very little animals moving in a very lively fashion. "They were the first bacteria the human eye ever saw." [See also “L.,” infra.] Nature, XXIX. (1883) p. 154.

COLT, J. B.-Determination of the Foci of Lenses.

U.S.A. Patent, No. 288025, of 17th September, 1883. COOMBS, C. P.-Address as President of the Postal Microscopical Society, 11th October, 1883.

[On "examining occasionally the food we eat or the clothes we wear."] Journ. of Microscopy, III. (1884) pp. 1-7.

Cox, J. D.-A new form of Microscope stand with concentric movements. [Post.]
Proc. Amer. Soc. Micr., 6th Ann. Meeting, pp. 147-8 (1 fig.).
Discussion, pp. 235-6.

D., E. T.-Graphic Microscopy.
[Description of coloured lithograph of Tingis Crassiochari.]

DARLING, S.-Micrometer.

Sci.-Gossip, 1884, pp. 1–2 (1 pl.). U.S.A. Patent, No. 287420, of 1st March, 1883.

D., E. T.-Drawing from the Microscope.

[Points out the error of B. Hobson's suggestion-Vol. III. (1883) p. 725— of a semi-rotation of the stage to cure the inversion with the neutral tint reflector. Also remarks on the value of the camera lucida: "In microscopical work the camera lucida is merely a preliminary adjunct of limited utility in determining proportions; no graphic or perfect drawing is helped by its continued use; after affording the barest outlines and positions the instrument becomes an encumbrance, and those who are practised in its employment feel a palpable sense of relief, and breathe again, when it is got rid of, to settle down to the earnest work of direct vision from the Microscope."]

DEAN, A.-Microscopical.

Sci-Gossip, 1883, pp. 265-6 (1 fig.).

[Description of a "micro-magic lantern" with or without camera lucida.] Engl. Mech., XXXVIII. (1884) p. 391 (1 fig.). DETMERS, H. J.-Resolution of Amphipleura by sunlight, mirror-bar central; with letters from R. B. Tolles and A. Y. Moore.

The Microscope, III. (1883) pp. 197-201 and p. 221. DICKENSON.-Art of photographing microscopic objects.

[The apparatus consists of (1) an inexpensive magic lantern, illuminated by a triplex petroleum lamp with the ordinary combination of lenses, and an extra tube with a small bull's-eye condenser; (2) a Microscope, placed horizontally, without the eye-piece; and (3) a frame to hold the glass screen for focusing the image, and to receive the sensitized plate when photographing. The period of exposure is from eighteen seconds to two hours.]

Dinner, Microscopists at.

Note read before Academy of Medicine in Ireland.
Engl. Mech., XXXVIII. (1883) p. 279.
Sci.-Gossip, 1884, p. 17.

[Facetious account of a mythical dinner at which "every article of food was

carefully examined."]

DIPPEL, L.-Ein verstellbares Zeichen pult. [Reported as from Lab. Hist. Collège de [See Vol. III. (1883) p. 565.]

The Microscope, III. (1883) p. 233. (An adjustable drawing desk.) France, 1883, p. 188, instead of 1879.

Bot. Centralbl., XVII. (1884) pp. 62-3 (2 figs.).

Eye-pieces, Report of the Committee on. [Vol. III. (1883) p. 711.]

Proc. Amer. Soc. Micr., 6th Ann. Meeting, pp. 175-7. Discussion, pp. 238-9. FISCHER, G.-Ueber einige Versuche zur Hebung der Chromatischen Aberration dioptrischer Fernrohre. (On some attempts to remove the Chromatic Aberration of dioptric Telescopes.)

[Contains an abstract of S. Merz's article "Ueber Dispersionsverhältnisse optischer Glaser " (Vol. II. (1882) p. 565), with additional remarks. Also report of letter from K. W. Zenger on his Endomersion Objectives, ante, Vol. III. (1883) p. 596, and post.]

GRIMM, J.-See Cohen, E.

Central-Ztg. f. Optik u. Mech., IV. (1883) pp. 265-7.

HAGER, H.-Le Microscope. Théorie et Application. (The Microscope. Theory and application.) Translated from the 4th German edition with annotations by L. Planchon and L. Hugounenq. Introduction by J. E. Planchon. pp., 350 figs. 18mo, Paris, 1884.

264

X. and

HAMMOND, A.-Address on resigning the chair of the Postal Microscopical Society.

[Account of the notes written by members of the

circulated.]

HILGARD, Prof.---See Micrometer Scale.

Society on the slides

Journ. of Microscopy, III. (1884) pp: 7-17.

Ser. 2.-VOL. IV.

HITCHCOCK, R.-Notes from Abroad.