native grass, although its vicinity is worthy of close examination as likely to lead to some interesting information respecting the time and manner of its introduction.

The list of mosses, as given in the Liverpool Flora, is very imperfect, arising in great measure from the fact that it was principally prepared by two botanists who had then only for a year or two been engaged in the study of this interesting class. Since its publication, there has been a large number of species added to it, some of them the rarest mosses we have, so that now there is an addition of at least half as many as the original list boasted. For these we are, in great measure, indebted to the indefatigable zeal of one of our local botanists, Mr. F. P. Marrat, who has himself been instrumental in searching out and naming some of the most obscure of our mosses.

Perhaps there has been no place more fruitful in the production of mosses than the sand hills of the Mersey. From Waterloo to Southport we have a succession of interesting species, many of them confined to the sea side, and well remunerating the muscologist for the trouble of collecting.

It is not my intention to occupy your time by any lengthy observations on the species or localities, but I may perhaps be permitted to make a few remarks on one or two of the mosses found on the sand hills.

Hypnum nodiflorum. A moss that appears in this locality to have been overlooked for years as a variety of H. stellatum, was first discovered to be distinct by Mr. Harrison of the Botanic Gardens. It is easily distinguished from H. stellatum, under the microscope, by the nerved leaf, that of H. stellatum being nerveless.

Hypnum elodes and lycopodioides, two rare mosses, have also been found growing plentifully on the sand hills-the former at Southport, and the latter at Crosby.

Among the Brya many interesting additions have been made from the same locality. Bryum Marratii was first found, by Mr. Marrat, on the flat sands at Southport, and was named after him by Wilson and Hooker. At first it was mistaken for a variety of B. calophyllum, but the true B. calophyllum or latifolium being found in company with it, led to a close examination of the supposed variety, and essential marks of distinction were discovered; the chief difference being in the form of the

capsule that of Marratii being nearly round and much smaller than in B. calophyllum, there is also a marked difference in the leaves. I may remark that B, calophyllum is by far the more rare of the two, being found in this one locality in very small quantities.

Bryum Warnerii, intermedium, rostratum, and turbinatum, have also been found at Southport.

Among the inland mosses there have also been many valuable additions to the Flora. Hypnum, trichomanes, striatum, pratense, loreum, sericeum, pumillum, aduncum, uncinatum, and revolvens, with others of less moment, have been added to the list.

Campylopus torfaceus, Didymodon heteromallum, Catoscopium nigritum, and Phascum serratum, with others too numerous to mention, all tend to make our list of species as interesting as that of the most favourite muscological districts.

ON THE ILLUMINATION OF THE DIATOMACEE, WHEN VIEWED UNDER THE MICROSCOPE.

By Thomas Sansom, A.L.S., F.B.S.E., &c.

(READ 24TH MAY, 1855.)

In bringing before the Society the subject of the illumination, under the microscope, of lined objects, or, in other words, the Diatomaceæ, by means of oblique light, I shall not enter into the theory of optics, but confine my remarks to a few practical suggestions on the best mode of obtaining a simple and efficient illumination when high powers are used.

Within the last few years, and especially since the establishment of the Microscopical Society of London, in 1840, the energies of several of the most talented opticians, both there and elsewhere, have been devoted almost exclusively to the improvement of the microscope; and that instrument is now so complete, mechanically and optically, that the microscopes of the three principal makers have obtained, as they are entitled to, universal celebrity.

The greatest of all the modern improvements, is, perhaps, the large increase made to the angle of aperture of the more recent object glasses,

and the three principal makers, viz., Mr. Andrew Ross, Messrs. Smith and Beck, and Messrs. Powell and Leyland, have produced most extraordinary glasses in this respect. The Jurors, in their report on the Great Exhibition of 1851, speaking of the glasses of Messrs. Smith and Beck, say "they are beautifully corrected for spherical aberration, but the secondary speculum has not been much diminished. The half inch focus of 70° aperture [now called four tenths] is a wonderfully fine combination, easily showing objects, considered difficult for a inch focal length a little more than a year since, and bearing the application of the higher eye pieces in an unprecedented manner."

Since the report was written, of which the above is an extract, other and still greater improvements have been made, and we have now inch object glasses of 85° of angular aperture,* of 150°, and ' of 170°.

Having thus attained such comparative perfection in our object glasses, it became necessary to improve the means of illumination, hence innumerable achromatic condensers have been invented, until we have now one for almost every class of objects in our cabinets. Many of these instruments are most elaborate and elegant as works of art, but they are costly, and are therefore very popular amongst a class of microscopists who use their instruments only to observe such characters as are already pointed out, and to be found on slides, sold mounted ready for exhibition.

In viewing the Diatomaceæ under the microscope, it is found from experience, that in order to see the markings on the more delicate shells, glasses of large angular aperture are required, and that the shell should be illuminated with oblique light. The question then for consideration is, what is the best mode of applying that light?

The principle involved in the construction of almost all the modern achromatic condensers is that of stopping out the central rays of light, and illuminating the object by the external rays only. This is usually accomplished by a revolving diaphragm, placed below the bottom lens of the combination, or introduced between the glasses. It is supposed by this means that shadows are avoided, and the definition of the object is improved. Now this must be admitted in theory to be correct, but I think it very doubtful whether in practice such perfection is often attained.

* I am now informed (July, 1855) that Mr. Ross has succeeded in producing inch object glasses, of 95° and 130° of angular aperture.

If you use an object glass, say a quarter of 240 linear magnifying power, and 72° of angular aperture, and from which the central rays have been stopped, under the stage as an achromatic condenser, and take for an object glass a low power, say of an inch focus, you will, on centering the condenser carefully, obtain a small cone of light, of about 20-1000th of an inch in diameter, in the centre of the field of view; then place on the stage of the microscope a micrometer, in lieu of an object, which, when properly focussed, will be viewed with light reflected directly through the microscope. Now if the mirror be kept in the same axis, as originally adjusted, but slightly elevated either on the right or on the left, the cone of light by which the object would be illuminated, will travel over a distance of 40-1000th of an inch on the micrometer, thus showing the extreme difficulty in adjusting a stopped condenser, for the higher powers, in such a way that all the external rays shall converge to a true centre. The whole field of view of Ross's object glass, with the lowest eyepiece is 12-1000th of an inch, whereas a very slight movement of the mirror will alter the position of the cone of light reflected through the condenser, and shown on the micrometer to the extent of 40-1000th of an inch, or more than three times the diameter of the whole field of view.

Under these circumstances, I conclude the advantages resulting from the use of stopped condensers is more ideal than real, and that most frequently when the objects are shown very successfully by means of these condensers, the illumination has been effected by unintentionally shutting off the rays of light from one side of the condenser, as well as from the centre. However, under any circumstances, I feel satisfied that the most important discoveries have been made by the application of oblique light, applied directly upon the object by means of the bull's eye, or the condenser recommended by Mr. Sollitt, (vol. iii. page 87 Journal of Microscopical Science). This condenser consists of two achromatic lenses, one of four, and the other of two inch focus. The four inch lens has an aperture of 1 inch, and the two inch lens an aperture of of an inch; they are placed at 12 inches asunder, and the compound focus is an inch beyond the smaller lens.

The readiest mode of using oblique light is to place the lamp directly opposite to the stage of the microscope, then apply an ordinary bull's eye, or the combination of lenses invented by Mr. Sollitt. By depressing or

elevating the body of the microscope, you may throw a flood of light on the object, at a greater or less angle, according to the power of the object glass. If the objective be of small angular aperture, and the light be applied at a greater angle than is suitable to the glass, the field will appear dark, and nothing will be visible; but should the objective be of large angular aperture, and of high magnifying power, the markings on the most difficult test objects will be clearly defined. Much of the success of the operation will depend on the position of the shell. In some cases, and especially in those where the markings are very delicate, it will frequently be necessary to revolve the slide; this may be done where the stage of the microscope has a circular motion, which is usual in the better class of instruments, but where that is not the case the same object may be attained by altering the position of the condenser, according as circumstances may dictate.

The best way to judge of the merits of condensers, is to ascertain what success has attended their use, and if the principle be recognised as a test of quality, I think it may be fairly assumed that next to Sollitt's condenser, the bull's eye is the best; unquestionably nearly all the discoveries have been made by the aid of one or other of these instruments.

The Hull microscopists have, until within the last few months, used a small bull's eye, which costs 7s. 6d., and to them we are indebted for many of our greatest discoveries. Messrs. Harrison and Sollitt, in a paper read before the meeting of the British Association at Hull, in 1853, have given us a short history of the discoveries made by them, and as their investigations have, until lately, been carried on with the aid of the bull's eye, I take leave to quote their remarks. Speaking of the Diatomacea they say, "we in Hull first discovered the delicate markings on their silicious coverings, and pointed them out to others as the proper test for lenses. The first of the Diatomacea on which the lines were seen was the Navicula Hippocampus of Ehrenberg, Pritchard, and Queckett. This discovery was made early in 1841, when specimens were sent to the Microscopical Society in London, but the London microscopists not being able to bring out anything but the longitudinal markings, a remark was published by Mr. Harrison, in the Microscopical Journal for June, 1841, stating that we had discovered cross striæ on the N. Hippocampus, but that these were only visible on some of the specimens. On this they were immediately written to, and