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On the Use of the Microscope on board Ship.¹

The following remarks embody the experience of Mr. Charles Darwin, F.R.S., on this subject, the importance of which increases as the science of zoology advances.

The facility in examining the smaller invertebrate animals, either alive or dead, depends much more on the form of the microscope used than would be at first expected. The chief requisite of a simple microscope for this purpose is strength, firmness, and especially a large stage; the instruments generally sold in this country are much too small and weak. The stage ought to be firmly soldered to the upright column and have no movement; besides the strength thus gained, the stage is always at exactly the same height, which aids practice in the delicate movements of the hand. The stage should be able to receive saucers, three inches in internal diameter. A disc of blackened wood, with a piece of cork inlaid in the centre, made to drop into the same rim which receives the saucers, is useful for opaque and dry objects: there should also be a disc of metal of the same size, with a hole and rim in the centre to receive plates of glass, both flat and concave, in diameter one inch and a half, for dissecting minute objects; a plate of glass of three inches diameter lets in too much light and is otherwise inconvenient. Close under the stage there should be a blackened diaphragm, to slip easily in and out, in order to shut off the light

¹ See Darwin to Owen 26 March 1848 in Correspondence vol. 4 and More Letters, vol. 1, pp. 59-60.

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completely; in this diaphragm there may be a small orifice with a slide, to let in a pencil of light for small objects. The whole microscope should be screwed into a solid block of oak, and not into the lid of the box as is usual.

The mirror should be capable of movement in every direction, and of sliding up and down the column; on one side there must be a large concave mirror, and on the other a small flat one; these mirrors ought to be fitted water tight in caps, made to screw off and on; and two or three spare mirrors ought undoubtedly to be taken on a long voyage, as salt water spilt on the mirror easily deadens the quicksilver. A small cap is very convenient to cover the mirror when not in use, and often saves it from being wet. The vertical shaft by which the lenses are moved up and down should be triangular (as these work much better than those of a cylindrical form), and there should be on both sides large milled heads; with such, there is no occasion for fine movements of adjustment, which always tend to weaken the instrument. The horizontal shaft should be capable of revolving, and should be moved to and fro by two milled heads (for the right and left hands), but the left milled head must be quite small, to allow of the cheek and eye approaching close to the lenses of high power. The horizontal shaft must come down to the stage.

The most useful lenses are doublets of 1 inch and 6-10ths of an inch (measured from the lower glass of the doublet) in focal distance; a simple lens of 4 or 5-10ths of an inch is a very valuable power; and, lastly, Codrington lenses¹ (of the kind sold by Adie of Edinburgh),²

¹ Mistake or misprint for 'Coddington lenses' which are made of a single lens with a grooved diaphragm around the circumference which allows for sharp images at higher magnification. Named after Henry Coddington (1798/9-1845), mathematician and clergyman, Tutor at Trinity College, Cambridge, 1822-1833.

² Alexander James Adie (1775-1858), optician and instrument maker.

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of 1-10th, 1-15th, and 1-20th focal distances, have been found most useful by two of the most eminent naturalists in England. With a little practice it is not difficult to dissect under the 1-10th lens, and some succeed under the 1-20th. A person not having a compound microscope might procure a 1-30th of an inch Codrington lens. All the lenses (except the largest doublet) should be made to drop, not screw, into the same ring; the large doublet may slip off and on the opposite end of the horizontal shaft. The best saucers have a flat glass bottom, with thin upright metal sides (silvered within); there should be at least four of them, being in depth (inside measure) 3-10ths, 5-10ths, 7-10ths, and a whole inch. Circular discs of fine-textured cork, of the size of the saucers (with one or two circular springs of steel-wire to keep the cork at the bottom of the water), serve for fixing objects to be dissected by direct instead of transmitted light. For this end short fine pins and lace-needles should be procured; wherever it is possible, the animal ought to be fixed to the cork under water. Of the smaller plates of glass of an inch and a half in diameter, some should be flat and some slightly concave; the latter are very useful—saucers of this small diameter are inconvenient.

The simplest and most useful instruments for minute dissection are the triangular glove-needles, which with a little cotton-wool and sealing-wax can be easily fixed into pieces of large-bored thermometer tubes; a stock of tubes and needles should be taken on a voyage. With these needles (by keeping the object only just immersed in a drop of water, which can be regulated by the suction

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of blotting-paper), wonderfully minute objects can be dissected; needles bent at their tips are convenient for some purposes. Arm supports are useful in minute dissections; two blocks of wood with inclined surfaces, coming up a little below the level of the stage, and resting partly on the stand of the microscope, can be made by a common carpenter. As it is often rather dark in the cabins of ships, a large bull's-eye glass on a stand (such as are sold with most compound microscopes) would be most useful to condense the light from a lamp on an opaque object, or to increase it when transmitted. Besides the needles, fine pointed forceps, pointed scissors, and eye scalpels are requisite. The French use an instrument called a microtome, and consider it most useful; others prefer finely pointed scissors, with one leg long and thick, to be held like a pen, and the other quite short, to be pressed by the fore finger, and kept open by a spring. A live-box to act as a compressor, or still better a proper compressor closed by a screw, and both made to drop into the rim of the stage, are valuable aids for making out the structure of transparent animals or organs. The observer should be provided with three slips of glass, or still better with three circular plates, made to drop into the stage of his microscope, and graduated into tenths, hundredths, and thousandths of an inch, to serve as micrometers, on which to place and measure any object he is examining. Some watch-glasses are very useful as temporary receptacles for small sea-animals. Minute parts after dissection can be preserved for years in very weak spirits of wine, by covering them, when placed on slips of glass, by small portions of very thin

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glass (both sold for this purpose), and cementing the edges with gold-size.*

When time and opportunity concur for the anatomical examination of an animal, the following notes or heads of observation will guide the dissector to the facts which it is most desirable to determine and note down.

No.			Date		18
Notes of Dissections performed at
	Animal's Name
Sex			Age		Weight
Length of body, from extremity of jaws to root of tail
—— of head				of tail
Situation of testes
——— of preputial orifice
——— of vaginal orifice
——— of anus
——— and number of mammæ
Abdominal muscles
——— ring
Stomach	simple	length		greatest circumference
		Observations.
	complex	number of sacs		relative size
		Obs.
Omentum
Mesentery
Intestines	length		greatest circumference
	—— of small			—— of small
	—— of cæcum			—— of cæcum
	—— of large			—— of large
	Observations
Anus			glands

————————————————

* A microscope such as here described, and most of the apparatus, can be seen at Messrs. Smith and Beck's, opticians, of Colman Street, London.

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Cloaca
Liver	situation
	number of lobes
	weight
	Observations
Gall-bladder,	size		situation
—————	structure
Bile, enters intestine
Pancreas	form
	situation
	its secretion, enters intestine
Spleen	situation
	form
	weight
Lungs	situation
	length		breadth, right		left
	weight
	number of lobes, right				left
	structure, air cells, &c.
Branchiæ
Heart	situation
	weight
	length		breadth
	shape and structure
Venæ cavæ
Aorta, primary branches
Trachea, number of rings			structure
Larynx
Pharynx
Epiglottis
Thyroid Glands
Salivary glands
Tongue, length			papillæ
Nostrils
Eye-lids
Eye
Pupil, form
Lachrymal gland
Ear
Brain, weight			form, &c.
Spinal cord, length
Supra-renal glands

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Kidneys	situation
	form		length		breadth
	weight of both
	papillæ, number and form
Ureters terminate
Urinary bladder	situation
	size
	shape
Testes	size
	structure
Vasa deferentia terminate
Vesiculae seminales		size
		structure
		terminate
Prostate	size
	structure
	terminate
Cowper's glands	size
	structure
	terminate
Penis	length		muscle
Urethra
Ovaries	situation
	size
	shape
	Observations
Uterus	length of cornua
	—— of Fallopian tubes
	—— of body
	position
Vagina
Oviduct	length
	form
	termination
Peculiarities of muscles
———— air-sacs
———— glandular organs
Morbid appearances
Calculi
Entozoa
Epizoa