1 My earliest acquaintance with the Microscope occurred in the thirties, when I fortunately became possessed of a Culpeper-Scarlet instrument, figured in the title-page.

2 At the time this was written, scarcely a book of the kind had been published at a price within the reach of the student.

3 For fuller information, see the Cantor Lectures on the Microscope, by the late John Mayall, F.R.M.S., “Society of Arts Journal,” 1885.

4 “A Practical Treatise on the Use of the Microscope.” London, 1855.

5 For further information, I must refer my readers to Parkinson’s “Treatise on Optics;” Herschel’s “Familiar Lectures on Light;” “CyclopÆdia Britannica;” Everett’s translation of Deschanel’s “Physics;” and NÄgeli and Schwendener’s “Theory and Practice of the Microscope,” translated by Frank Crisp, LL.D.

6 The cornea of the eye is not so entirely the simple transparent structure as it at first sight may appear to be. It is composed of several layers, the most important of which is the nerve layer, consisting of innumerable ganglionic stellate plexus of cells held together by a network, as seen in Fig. 21, a small section stained by chloride of gold, and magnified 300 diameters. Beneath the nucleated nerve cells is a second layer of stellate cells, varying a little in their form. These nerve and stellate cells serve the purpose of maintaining the cornea in health, and must play a significant part in the dioptric system.

7 The standard condition of perfect vision is termed emmetropia.

8 Landolt; “The Accommodation and Refraction of the Eye,” 1886.

9 µ = ·001 of a millimetre. This measurement is now universally employed in microscopy.

10 Diffraction effects may be observed without a microscope, indeed, the more striking are seen in connection with telescopic vision. A beautiful series of phenomena in illustration of the diffraction of light may be produced as follows: Draw on a large sheet of paper a series of geometrical figures, arranged at equal distances in a circle. A collodion photographic picture of these being taken, a series of small transparent apertures in the elsewhere opaque film will result. This film is then mounted, so that it may be in turn brought before the centre of a small hand telescope, previously adjusted to view an image of the sun. In this way we have an apparatus of the most compact form, and by means of which a series of fifty or more phenomena may be brought into view in a few minutes. These pictures being very small (occupying on an average area one-tenth of an inch in diameter), inaccuracies of surface and substance of the glass may be neglected. A film of Canada balsam with which the glass is cemented over the picture produces no disturbance. There is a manifest advantage in the figures being small, as the size of the image is in inverse proportion to the size of the aperture.

11 Carpenter, “The Microscope,” p. 65, 1891.

12 “Phil. Mag.,” viii., p. 167 (1896).

13 Professor Stokes wrote me in the following flattering terms:—“What you have submitted to me on the subject of apertures is so sound, clear, and succinct, that I have nothing to add to it. The method adapted as you have explained respecting the immersion system, I consider to be perfectly satisfactory.” Subsequently, and at my request, Sir George Stokes contributed a valuable paper on the subject to the “Transactions of the Royal Microscopical Society,” 1876, on “The Theoretical Limit of Aperture.”

14 “On the Estimation of Aperture in the Microscope,” “Journal of the Royal Microscopical Society,” series ii. vol. i.; “Notes on Aperture, Microscopic Vision, and the Value of Wide-angled Immersion Objectives,” 1881.

15 Numerical aperture is generally used in the sense in which it was introduced in 1873 by Professor Abbe, on the basis of his theoretical investigations. Numerical aperture represents the ratio between the radius of the effective aperture (p) of the system on the side where the image is formed—more accurately the radius of the emerging pencils measured in the upper focal plane of the objective—and the equivalent focal length (f) of the latter, i.e.,

Numerical aperture = p/f.

This ratio is equal to the product of the sine of half the angle of aperture u of the incident pencils and the refractive index n of the medium, situated in front of the objective. With dry lenses n has therefore the value 1; with immersion lenses it is equal to the refractive index of the particular immersion fluid:

Numerical aperture = n Sin u.

The numerical aperture of a lens determines all its essential qualities; the brightness of the image increases with a given magnification and, other things being equal, as the square of the aperture; the resolving and defining powers are directly related to it, the focal depth of differentiation of depths varies inversely as the aperture, and so forth. (Abbe, “The Estimation of Aperture,” “Journal of the Royal Microscopical Society,” 1881, p. 389.)

16 “Journal of the Royal Microscopical Society.”

17 “Journal Roy. Micros. Soc.,” p. 19, 1878, and p. 20, 1880.

18 “The Magnifying Power of Short Spaces” has been ably elucidated by John Gorham, Esq., M.R.C.S. “Journal of Microscopical Society,” October, 1854.

19 The late Mr. Coddington, of Cambridge, who had a high opinion of the value of this lens, had one of these grooved spheres executed by Mr. Carey, who gave it the name of the Coddington Lens, supposing that it was invented by the person who employed him, whereas Mr. Coddington never laid claim to it, and the circumstance of his having one made was not known until nine years after it was described by Sir David Brewster in the “Edinburgh Journal.”

20 “Journal of the Royal Microscopical Society, 1890,” p. 420.

21 “Journal of the Royal Microscopical Society, 1880,” p. 1050.

22 Apo-chromatic, from the Greek, signifying freedom from colour.

23 Prof. Abbe “On Stephenson’s System of Homogeneous Immersion for Microscope Objectives,” “Journal of the Royal Microscopical Society,” II. (1879), p. 256, and on “The Essence of Homogeneous Immersion,” Ibid., I. (1881), p. 131.

24 Reichert, in his catalogue, does not clearly indicate what the initial powers of his eye-pieces are.

25 Messrs. Ross have two series of eye-pieces, both Huyghenian. One series is for use with the English 10-inch tube-body, and is distinguished by Roman letters, and the other by numerals, and made as is usual on the Continent, and for use with the shorter tube-body 6½-inch. The initial powers given in the table are for the 10-inch tube, and for the shorter must be read as follows:—

26 This centring-glass consists of a tubular cap with a minute aperture, containing two plano-convex lenses, so adjusted that the image of the aperture in the object-glass and the images of the aperture of the lenses and the diaphragms contained in the tube which holds the illuminating combination, may be all in focus at the same time, so that by the same adjustment they may be brought sufficiently near to recognise their centricity.

27 Summary of the value of parabolic illumination and immersion illuminators, by the late Mr. J. Mayall, will be found on p. 27, “Journal of the Royal Microscopical Society” (1879).

28 Messrs. Baker and Swift have constructed lamps with removal and fixed achromatic bull’s-eye lenses in gymbal, and changeable tinted glass screens. Either of these will add to the usefulness of the lamp in bacteriological research work. Baker’s is constructed on the Herschel doublet formula, and should therefore be free from aberration. It is mounted on a heavy brass tripod foot, has vertical and horizontal movements by rack and pinion, brass reservoir, with screw opening for filling, metal chimney to take 3 × 1½-inch glass slip, removable frame for carrying tinted glass screens, &c.

29 “Journal of the Royal Microscopical Society,” p. 365, 1896.

30 Dr. G. A. Piersoll, “American Annual of Photography,” 1890.

31 “Journal of the Royal Microscopical Society,” 1892, p. 684.

32 “Journal of the Royal Microscopical Society,” p. 578, 1897.

33 Herapath’s test-fluid is a mixture of three drachms of pure acetic acid, one drachm of alcohol, and three drops of sulphuric acid.

34 “Journal of the Royal Microscopic Society,” 1867.

35 Born in 1787, at Straubing, a small town in Bavaria.

36 Dr. Thudicum’s “Tenth Report of the Medical Officer of the Privy Council, 1867.” Mr. Sorby “On Some Improvements in the Spectrum Method of Detecting Blood.” “Journal of the Royal Microscopical Society,” 1871.

37 “On the Reduction and Oxidation of the Colouring-matter of the Blood” (“Proc. of the Royal Soc.” vol. xiii. p. 355). The oxidising solution is made as follows:—To a solution of proto-sulphate of iron, enough tartaric acid is added to prevent precipitation by alkalies. A small quantity of this solution, made slightly alkaline by ammonia or carbonate of soda, is to be added to the weak solution of blood in water.

38 “Journal of the Royal Microscopical Society,” 1869.

39 Professor Sylvanus Thompson, “On the Measurement of Lenses,” “Journal of the Royal Microscopical Society,” 1892, p. 109.

40 “Journal of the Royal Microscopical Society,” 2nd Series, Vol. iv., p. 542.

41 Mr. J. F. Smith, “On the Structure of the Valve of Pleurosigma Pellucida,” “Quekett Club Trans.”

42 “Quarterly Journal of Microscopical Science,” New Series, Vol. viii., 1878.

43 It is quite possible also for the student to make his own microscope stand. Mr. Field in the “English Mechanic,” pp. 171 et seq., 1897, furnishes numerous working drawings for the construction of a high-class stand, together with patterns for the metal work.

44 “Modern Microscopy,” by Martin J. Cole.

45 With regard to the use of absolute alcohol, this re-agent requires to be used with caution; all minute details are lost, and it causes irregular shrinking of the finer tissues, while fibrous tissue is brought into undue prominence at the expense of the cellular elements. Consequently in certain biological laboratories the method of hardening in alcohol has been abandoned in favour of other re-agents.

46 “Journal of Anatomy and Physiology,” XX. 1881, p. 349.

47 “Journal of the Quekett Club,” July, 1893, and March, 1895.

48 Mr. John Hood, 50, Dallfield Walk, Dundee, offers a weekly supply of infusorial life for a small annual subscription, or a single tube by post at the trifling cost of one shilling.

49 Professor Marshall Ward, F.R.S., “Address to the Botanical Section of the British Association, 1897.”

50 “British Medical Journal,” March 26, 1859; “Medical Times and Gazette” and “Popular Science Review,” 1862.

51 “Parasitic Diseases,” “Journ. of the Royal Micros. Soc. of Lond.,” 1859-60.

52 There are several other kinds of bacteria infesting milk, some of which are motile, others non-motile, producing acidity and colouring matter, as B. prodigiosus, red-milk; B. synxanthus, yellow milk; B. lactis aerogens, which are pathogenic; B. lactis albus, which coagulate milk; and another form, which is productive of slimy or ropy-milk.

53 “Parasitic Diseases of the Skin,” 1859-73, p. 30. Bailliere, Tindal, and Cox.

54 “Organic Germ Theory of Disease,” “Medical Times and Gazette,” p. 685, 1870.

55 F. Cohn on the “Natural History of Protococcus pluvialis.”

56 Pritchard’s “Infusoria,” p. 24, Plate I., 4th edition.

57 In order to detect the presence of starch-grains in plants, the tissue must be kept in alcohol exposed to light, until the whole of the chlorophyll is dissolved out; it must then be treated for several hours in a strong solution of potash. After neutralisation with acetic acid, the tissue may be treated with iodine, which colours it blue, or with coralline solution, which colours it pink.

58 Verhandl. d. Natur. Hist. Jahr. xx. p. 1. “Micros. Jour. Science,” vol. iii., p. 120.

59 For instance, where the yellow Palmella is found the Chlorococcus will assume a yellow tinge in its soridial stage. Viewed by transmitted light the sori are seen as opaque balls, with an irregular outline.

60 “Contributions to the Knowledge of the Development of the Gonidia of Lichens.” By J. Braxton Hicks, M.D., “Quarterly Journal of Microscopical Science,” vol. viii., 860, p. 239.

61 Berkeley’s “Introduction to Cryptogamic Botany,” 1857.

62 For more detailed information on the structure and classification of unicellular plants, and cryptogams, the reader is referred to Ralfs’ “British DesmidaceÆ”; Smith’s “British DiatomaceÆ”; Goebel’s “Outlines of Classification and Special Morphology”; Berkeley’s “Cryptogamic Botany”; De Bary’s “Comparative Anatomy of the Phaneragams and Ferns”; Professor Marshall Ward’s “Sach’s Physiology of Plants,” and numerous memoirs on Fungi; and Bower and Sidney Vine’s “Course of Practical Instruction in Botany,” a most instructive book on the histology of plants.

63 “A Manual of the Infusoria,” by W. Saville Kent, F.L.S., &c., 1880.

64 “Journal of the Linn. Society,” vol. viii., p. 202; vol. ix., p. 147, 1865 and 1866.

65 Among the more important works on Foraminifera for consultation will be found D’Orbigny’s “Foraminiferes Fossiles du Bassin Tertiaire de Vienne” (Autriche); Schultze, “Ueber den Organismus der Polythalamien,” 1854; Carpenter and Williamson’s “Researches on the Foraminifera,” “Phil. Trans. 1856;” Parker and Rupert-Jones in the “Annals of Natural History.” Specimens of Foraminifera may be obtained by shaking dried sponges; but if required alive they must be dredged for, or picked off the fronds of living seaweeds, over the surface of which they are, by the aid of a lens, seen to move.

66 W. Saville Kent, F.L.S., Op. Cit., p. 335.

67 Difficulties formerly associated with the microscopic examination of flagellate forms of infusorial life have been overcome by improvements in the objectives, by the knowledge gained of the monad groups, and by the exhaustive researches of Drs. Drysdale and Dallinger, whose joint investigations were published in the Journal of the Royal Microscopical Society, 1873-75. By employing the highest and most perfectly constructed powers of the microscope, and devoting an enormous amount of time and attention to unravelling mysteries so long associated with the production of the lowly organised flagellate organisms, monads, and patiently watching hour by hour, the life-history of numerous species of these minute infusorial animalcules were obtained. Not only was it discovered that these organisms increased indefinitely by fission, but that under certain conditions two or more individuals were united into encystments, and whose contents broke up into a greater or less number of spore-like bodies, were speedily developed into the parent type. In the examination of these minute bodies, it has been found that talc-films, that is, talc split into extremely fine laminÆ, offer the best kind of cover, in fact, supersede ordinary glass covers, and possess an advantage, that of bending readily, thus permitting the objective to be brought close down upon the object.

68 R. Kirkpatrick, Warne, Op. Cit., pp. 532-3.

69 Saville Kent, op. cit., p. 191.

70 Fritz MÜller first demonstrated a nervous system in the Polyzoa:—“The nervous system of each branch consisting of—1st, a considerable sized ganglion situated at its origin; 2nd, of a nervous trunk running the entire length of the branch, at the upper part of which it subdivides into branches, going to the ganglia of the internodes arising at this part; and 3rd, of a rich nervous plexus resting on the trunk, and connecting the ganglia just mentioned, as well as the basal ganglia of the individual polypides.” For further account, see paper in the “Micros. Journ.,” vol. i., New Series, p. 330.

71 I have ventured to devote some considerable space to the development of the pond-snail, and for an obvious reason, that of making it perfectly clear to my readers that my microscopical investigations of Limnoea, made in 1853, were published in the “Journal of the Microscopical Society,” June, 1854, and republished in extenso in the several editions of this book, dating from the last mentioned period. Nevertheless, the fringe of cilia was, it appears, rediscovered in 1874, just twenty years after my paper was published. It is almost unnecessary to add that Carpenter gravely errs in his statement “that the existence of the fringe of cilia in the embryo snail had been overlooked until 1874.”

72 Mr. George Rainey many years ago made us acquainted with the fact that certain of the appearances presented by the shell or other hard structures of animals, and which had hitherto been referred to as cell-development, are really governed by the physical laws which govern the aggregation of certain crystalline salts when exposed to the action of vegetable and animal substances in a state of solution. Mr. Rainey furnished a process for obtaining artificially a crystalline substance which shall so closely resemble shell structure that it can barely be distinguished from it. The chemical substances to be used in the preparation of the artificial shell, or calculi, are a soluble compound of lime and carbonate of potash or soda, dissolved in separate portions of water, and mixed with some viscid vegetable or animal substance, as gum or albumen, and mixing the several solutions together. The mechanical conditions required are that such a quantity of each of the viscid materials in each solution shall be of about the same density as that of the nascent carbonate of lime, and at perfect rest. This state of rest will require from two to three weeks or longer. Mr. Rainey shows the analogy or identity of his artificially formed crystals with those found in natural products both in animals and vegetables, chiefly confining himself to the structure and formation of shells and bone, pigmental and other cells, and the structure and development of the crystalline lenses, which he contends are all formed upon precisely the same physical principles as the artificial crystals.

73 E. Ray Lankester, “On the GregarinÆ found in the common Earthworm.”—“Micros. Trans.” vol. iii. p. 83.

74 For the fullest information of marine, land, and fresh-water species, consult Dr. Bastian’s “Monograph on the AnguillulidÆ”; “Lin. Soc. Trans.” vol. xxv. p. 75; the “Anguillula Aceti,” by the author, in the “Popular Science Review,” January, 1863.

75 “Cercaria parasitic on Limnoea,” “Jour. Royal Micros. Soc.” 1870.

76 See my paper “The Natural History of a Nematode Worm,” “Journ. of Microscopy and Natural History,” October, 1888.

77 “The Parasites of Man and the Diseases which proceed from them,” by Professor Rudolf Leuckart, 1886.

78 R. J. Pocock, “On Worms” (Warne, Op. cit.), p. 465.

79 An interesting account of the formation of the tubes of Serpula is given by Mr. Watson, “Jour. Micros. Soc.,” vol. 1890, p. 685.

80 Dr. Baird, “Natural History of British Entomostraca,” printed for the Ray Society, 1850.

81 See Mr. B. T. Lowne’s exhaustive treatise on “The Anatomy and Physiology of the Blow-fly,” a volume of 750 pages and 52 plates, 1891.

82 Tuffen West, “Trans. Linn. Soc.,” vol. xxiii., p. 393.

83 The term micropyle (a little gate) has heretofore only been used in its relation with the vegetable kingdom: it is used to denote the opening or foramen towards which the radicle is always pointed.

84 Dr. Halifax adopts the method of killing the insect with chloroform; he then immerses it in a bath of hot wax, in which it is allowed to remain until the wax becomes cold and hard; with a sharp knife sections are easily made in the required direction without in the least disturbing any of the more fragile parts, or internal organs of the specimen.

85 “Phil. Trans.,” 1859, p. 341.

86 See my paper on “The Eggs of Insects,” in “The Intellectual Observer,” Oct. 1867, in which other varieties of eggs are given.

87 W. U. Whitney, “Transactions of the Microscopical Society” for 1861 and 1867.

88 Mr. F. G. Cuttell, 52, New Compton Street, Soho, cuts and prepares excellent sections.

89 Published with his paper in detail, “Aperture as a Factor in Microscopic Vision,” “Journal of Royal Micros. Soc.,” June, 1808, pp. 334 et seq.

90 “Squire’s Methods and FormulÆ;” “Modern Microscopy,” Cross and M. F. Cole; “The Microscopists’ Vade Mecum,” A. B. Lee; “Bacteriology.” Professor Dr. E. Crookshank, Messrs. Baird and Tattock, Cross Street, Hatton Garden, supply all Scientific Apparatus for Bacteriological Work.

91 The imperial gallon contains 277.27384 cubic inches, and the imperial pint 20 fluid ounces, whereas the wine gallon has 231 cubic inches and the pint 16 fluid ounces. In wine measure 1 litre = 33.815 fluid ounces.