patience and perseverance, before the cake can be brought to bear hard blows: but it may, by these means, at length be made so flat and square, as to bear being passed through the flatting-mill, and so laminated to any required degree of thinness.

Thus prepared, it is always brittle, while hot; possibly, from its still containing a small remnant of sulphur. I have also fused some palladium per se, without using sulphur; but I have always found it, when treated in this way, so hard and difficult to manage, that I greatly prefer the former process.

To obtain the oxide of Osmium in a pure, solid, and crystallized state, I grind together, and introduce, when ground, into a cold crucible, 3 parts by weight of the pulverulent ore of iridium, and 1 part of nitre. The crucible is to be heated to a good red in an open fire, until the ingredients are reduced to a pasty state; when osmic fumes will be found to arise from it. The soluble parts of the mixture are then to be dissolved in the smallest quantity of water necessary for the purpose, and the liquor, thus obtained, is to be mixed, in a retort, with so much sulphuric acid, diluted with its weight of water, as is equivalent to the potash contained in the nitre employed; but no inconvenience will result from using an excess of sulphuric acid. By distilling rapidly into a clean receiver, for so long a time as the osmic fumes continue to come over, the oxide will be collected in the form of a white crust on the sides of the receiver; and there melting, it will run down in drops beneath the watery solution, forming a fluid flattened globule at the bottom. When the receiver has become quite cold, the oxide will become solid and crystallize. One such operation has yielded 30 grains of the crystallized oxide, besides a strong aqueous solution of it.

II. A description of a microscopic doublet. By WILLIAM HYDE WOLLASTON, M.D. F.R.S. &c.

Read November 27, 1828.

THE state of my health induces me to commit to writing, rather more hastily than I have been accustomed to do, some observations on microscopes; and I trust, that in laying them before the Royal Society, they will meet with that indulgence which has been extended to all my former communications.

In the illumination of microscopic objects, whatever light is collected and brought to the eye, beyond that which is fully commanded by the object-glasses, tends rather to impede than to assist distinct vision.

My endeavour has been, to collect as much of the admitted light as can be done by simple means, to a focus in the same plane as the object to be examined. For this purpose I have used with success a plane mirror to direct the light, and a plano-convex lens to collect it; the plane side of the lens being towards the object to be illuminated.

With respect to the apparatus for magnifying, notwithstanding the great improvements lately made in the construction of microscopes, by the introduction of achromatic object-glasses, and the manifest superiority they possess over any single microscope, in the greater extent of field they present to view at once, whereby they are admirably adapted to make an entertaining exhibition of known objects, hardly any one of the compound microscopes which I have yet seen, is capable of exhibiting minute bodies with that extreme distinctness which is to be attained by more simple means, and which is absolutely necessary for an original examination of unknown objects.

My experience has led me to prefer a lens of a plano-convex form, even when made of glass; but the sapphire lens of this form, recently introduced into use by Mr. PRITCHARD, has a decided superiority over every single lens hitherto employed.

The cost, however, of such a lens in comparison with glass, as well as the readiness with which any number and variety of the latter kind can be procured, led me to consider what simple combinations of them might perhaps equal the sapphire lens in performance, without great cost, or difficulty of construction; and though both Mr. HERSCHEL and Professor AIRY have recently applied their superior talents to the analytical investigation of this subject, it seemed not impossible that the more humble efforts of a mere experimentalist, might be rewarded by some useful results.

The consideration of that form of eye-piece for astronomical telescopes called Huygenian, suggested the probability that a similar combination should have a similar advantage, of correcting both chromatic and spherical aberration, if employed in an opposite direction as a microscope.

The construction which I found convenient in my trials, may be not unaptly compared to two thimbles fitted one within the other by screwing, and each perforated at the extremity. By this construction, two suitable plano-convex lenses fixed in these perforations, may, because of their plane surfaces, have their axes easily placed in the same line; and their distance from each other may be so varied, by screwing, as to produce the best effect of which they are susceptible.

As far as my trials have hitherto gone, I am led to consider the proportion of 3 to 1 as nearly the best for the relation of the foci of these lenses; and their joint performance to be the most perfect, when the distance between their plane surfaces is about 1% of the shorter focus. But as all the lenses I possess are not similar segments of spheres, or of the same relative thickness, I could not expect exact uniformity in the results.

The following is a description of the apparatus which I have employed. T, U, B, E, (Plate II. fig. 1.) represents a tube about six inches long, and of such a diameter as to preclude any reflexion of false light from its sides; and the better to insure this, the inside of the tube should be blackened. At the top of the tube, or within it, at a small distance from the top, is placed either a plano-convex lens E, T, or one properly crossed, so as to have the least aberration, about three-quarters of an inch focus, having its plane side next the object to be viewed; and at the bottom is a circular perforation A, of about three-tenths of an inch diameter, for limiting the light reflected from the plane