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I have several much deeper made on this plan up to

inch-all

of which shew the parallel lines in question, and other equally difficult objects, the deepest lenses have their apertures somewhat larger than this ratio, for the sake of the light, (for it appears that you may increase this aperture, though you must not diminish it, and yet see these objects, though the fog then becomes very great and disagreeable; the colour also grows very apparent on account of the largeness of the aperture relative to its focus. Common microscopic object-glasses as we all know are sufficiently achromatic with the small apertures, and the shallow eye-glass of one inch focus usually employed, in which respect there is an analogy between them and telescopes with object-glasses composed of single lenses of small aperture, and a shallow eye-glass. The achromatics only differ from them in carrying a larger aperture with a deeper eye-glass, which again have their limits, beyond which the colour appears as before.

In Figs. VII, VIII, and IX, are representations of some silver cups for holding very deep single lenses intended to view opaque objects, which, together with the object-glasses before-mentioned, were executed for me by Mr. Tuther, optician, in High Holborn, to whose politeness and skill I am indebted for being able to carry my intentions into effect. It is generally supposed that single lenses will shew objects perfectly clear and without fog, but this is not the case unless their apertures are very small,-lenses of 30, 0, 0, andth of an inch focus require their apertures to be so much reduced to shew opaque objects clearly, that it is scarcely possible to see at all with them from the want of light. These cups were contrived to remedy this defect as far as it is practi

scope would be, having an object-glass of the same aperture with the microscopic one with a focal length, equivalent to the distance between the object-gluss of the microscope, and the focus of its eye-glass having its image magnified by the said eye-glass. For example, I measured the power of a microscope in the legitimate way with two similar micrometers, one on the stage, the other at the field bar in the focus of the eye-glass-supposing the eye-glass of 1 inch focus to have magnified six times, the power was 36, while the size of the pencil at the eye-glass compared with the diameter of the object-glass was merely as 2 to 6-the oue being of an inch, the other roo

cable; their radius is only of an inch, their focus consequently . These condense light much more than the larger cups commonly used, and illuminate much more powerfully. It is true that they only enlighten a small portion of an object, but then we can only see a very small portion with such deep lenses as they are intended to hold; they are not so small but that they may be made to receive and condense the whole of the light proceeding from a bull's-eye lens placed at a proper distance from them, and in this way with no other light than that of a common candle, I have been enabled to see well an opaque object with a compound microscope, having an object-glass of only inch focus set in one of them, with only a moderate aperture. A lens of inch set in this manner, used as a single lens, likewise shews opaque objects in a manner which leaves nothing to be desired.

I must mention, however, that it is necessary for the stops between which the lenses are placed to be very accurately made. They should be turned out of a piece of solid brass, the external one very thin, and the holes so correct as always to coincide with each other when the stops are turned round; the apertures must be quite free from burrs; in addition to which the stops must be so adjusted that the focus of the lens and that of the cup must precisely correspond, otherwise the benefit of the cup is in a great measure lost. Fig. IX will carry or of an inch without any stop at all, which is a great convenience, for the lens is in this case close to the eye, and the field of view larger in consequence: the stops for the deeper lenses are much shorter than they would be with larger cups, (Figs. VII and VIII,) so the field is increased in the same way, and the eye much less strained in using them than it would be were the lens farther off from it. I have shewn many individuals objects with the inch lens not remarkable for the strength of their eyes, who saw with perfect ease, and were not at all conscious of the extreme smallness and depth of the lens they were using. As single lenses are generally considered to be most adapted for making discoveries in natural history, as being less likely to create optical deceptions than compound magnifiers, I imagine I am doing naturalists a service in putting them into a

way of using very deep ones without destroying their eyes*. I humbly recommend the contents of this paper to opticians, without being at all ambitious to acquire the honour of teaching them their own profession. I have the highest consideration for their practical knowledge, and conceive that one ounce of it is worth a ton weight of that of a mere theorist; at the same time I hope they will accept of my apologies for pointing out a few circumstances to them, (certainly not of much importance,) which the value of their time and the multiplicity of avocations of higher consequence will not usually permit them to attend to. If what I have written shall prove of no service to them, it is quite clear that my labours have been utterly useless. Indeed, it is too much the case that the researches of amateurs only terminate in discovering something which was perfectly well known before, and which only therefore serves to shew their own shallow acquaintance with the subject, or in bringing forward something as an improvement which has been tried and rejected long ago by those practically versed in the mysteries of optics.

ART. IV. Hints on the possibility of changing the Residence of certain Fishes from salt water to fresh.-By I. MAC CULLOCH, M. D., F. R. S., &c.

IN the tenth volume of Tilloch's Journal, there is a paper on the means to be employed for multiplying fish, translated from one which appeared in the Moniteur, by Monsieur Nouel, of Rouen. Although the chief speculations of this writer, which are of a very

*They may, perhaps, also thank me for informing them that Mr. Cornelius Varley, of Upper Thornhaugh-street, Bedford-square, (the inventor of the graphic telescope,) worked the small lenses for me which I have described; they were polished on wax tools, the figure is as correct as that of any shallow lenses, and their image will bear magnifying perfectly well. Mr. Varley and Mr. William Tulley of Islington, are the only individuals I know who cau make such deep lenses as they ought to be made.

interesting nature, concern the means of transferring the inhabitants of fresh waters in one country, or those of certain lakes or rivers, to others where they are not found, some hints are also introduced respecting the possibility of rendering certain sea-fish inhabitants of fresh waters. The whole paper is highly worthy of attention; but I am not aware that it has been followed by any of the practical trials recommended by the author, on which its economical value must ultimately depend. An example in point which recently came under my notice in Shetland, has induced me to examine the subject with somewhat more care than the author of that memoir seems to have bestowed on it, and to inquire more minutely into the arguments on which the probability of success rests. The following seem to be the only results which have been obtained, or were previously known with respect to that part of M. Nouel's plan, which relates to the cultivation of sea-fish in fresh water.

The plaice, Pleuronectes Platessa, as it appears, has been carried from the North sea to the ponds of East Friesland, where it has become established. The herring is said by Liancourt to frequent the Potowmack, Hudson, Elk, and Delaware rivers; but it has not appeared that the author's project to take it from the Seine into fresh-water ponds has been put into practice. The authority of Twiss for the existence of this fish in the fresh water lakes of Ireland, is more than questionable, and M. Nouel is assuredly misinformed when he states that it is found in prodigious shoals in Loch Lomond and Loch Eck in Scotland, both of them fresh inland lakes. I know not how this author can have thus been misled, unless he has mistaken some of the sea lochs for fresh-water lakes; though he could scarcely have confounded those he has named with any of the western inlets. I shall hereafter, however, point out a fact which renders his assertion possible; though he could not have been acquainted with it, as it is not very long since it was known, and has not been published in any work likely to have reached his hands.

It is also asserted in the same paper, that the salmon, in Scotland, has, in certain lakes, become naturalized, " abandoning their erratic taste, for a calm and settled life." Whether such an experiment

might not succeed, by forcibly transporting the salmon to lakes from which they could not reach the sea, is yet to be tried; but certainly there are not at present any salmon found in the Scottish lakes, except where they have the power of making their annual migrations into salt water. That salmon are attached to the particular rivers where they have been spawned and bred, is believed by all the fishermen; but this does not prove that they are naturalized to those fresh waters, as they invariably return to the sea after having deposited their spawn.

According to Pallas, the sturgeon, the sterlet, and some species of salmon reside in the river Kama without ever descending to the Caspian sea; and the authority of such a naturalist is perhaps sufficient to establish this interesting fact.

These, then, are the whole of the proofs which, in M. Nouel's paper, are adduced in support of this project; it remains to be seen by what other facts and reasonings its plausibility may be supported, and an inducement offered to those who have it in their power, to make such experiments as alone can establish it among those facts. in natural history which are capable of being applied to the uses of man; to increasing the quantity, or adding to the accessible variety of his food.

In the first place, it must be remarked, that the habits of many sea-fish are, in this respect, so convertible, or so easily assimilated to the requisite change, that a large portion of their time is passed in fresh water. The common salmon, the grey salmon, and the salmon trout, Salmo Salar, Salmo Eriox, and Salmo Trutta, are familiarly known to frequent rivers for the purpose of spawning; returning to the sea when this operation has been performed. The Salmo Migratorius leaves the lake Baikal for the same season; and, with us, the S. Lavaretus, or Gwiniad, and the S. Eperlanus or smelt, also quit the sea; ascending rivers at the spawning season, as does the Salmo Autumnalis, an inhabitant of the frozen ocean.

Now though M. Nouel is wrong in saying that the salmon is found in the Scottish lakes excluded from access to the sea, it is a fact that the salmon trout, or sea trout, as it is called in Scotland, is now a permanent resident in a fresh-water lake in the

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