obtain with certainty illumination at definite angles with Microscope stands which are not fitted with a swinging substage. The inventor describes it as follows:

"A perspective view of the apparatus (slightly reduced in size) is shown in Fig. 5. It consists of a transverse bar of brass (1), at one end of which, attached by a hinge (2), is a square brass plate (3), which can be inclined at any desired angle. This plate is transfixed

centrally by a brass tube half an inch long, in which a second tube (4) an inch and a half long slips easily. The slip-tube (4) is provided at one end with the Society's screw, by which a 3-inch objective (5) or any other preferred for the purpose, can be attached. The movable square plate is provided with a spring catch (6) which fits into any one of a series of notches in the edge of a brass quadrant (7), and thus serves both to hold the plate in position and to register the angle of obliquity. The transverse bar (1) slips in a groove on the upper surface of a strong brass tube (8), fitted to the substage of the Microscope. The bar itself has a longitudinal slot running nearly its whole

length, so that it can be pushed to any desired position without disturbing the position of the central steel rod (10), at the upper end of which a lens (9) is fastened. The lens (9) is such a segment of a hemisphere of crown glass, that when brought into optical contact (by oil of cloves) with the under surface of an ordinary glass object-slip, the object to be studied will be as nearly as possible at its centre of curvature, and the rod (10) slips freely in the top of the substage tube (8), so that the lens may be pushed into position or withdrawn at pleasure.

In using this apparatus with monochromatic sunlight, I first set the square brass plate )3) at the desired angle as read on the quadrant, and then slip the transverse bar (1) backwards or forwards as may be necessary, until the pencil of monochromatic sunlight (to which the desired degree of obliquity has been previously given by means of a prism) falls centrally through the slip-tube (4) and illuminating objective (5) upon the face of the lens with which the object is viewed. By means of the slip-tube, the illuminating objective (5) is then brought to the proper focal position. Ordinary illumination is thus obtained of any desired obliquity, from about 30° to the limit of the thickness of the stage. When I desire still greater obliquity I use Powell and Lealand's extra stage, and slip the transverse bar into the groove at the upper end of the holder which those makers provide with it to carry the small bull's-eyes they furnish for the examination of Amphipleura pellucida. In this manner I can get more oblique illumination up to 80° or even 85°, but of course the oblique pencils thus obtained are refracted at the under surface of the glass slip that carries the object, and cannot possibly reach the object itself at an obliquity greater than 41°. To obtain greater obliquity than this, I make use of the hemispherical lens (9). The illuminating objective is set at the desired angle, say 45°, and the object illuminated as described above. When this is satisfactorily done a drop of oil of cloves is placed on the flat surface of the hemispherical lens, which is then pushed up into contact with the under surface of the slide on which the object is mounted. The light now enters in the line of a radius of the hemisphere, at the angle registered on the quadrant (7). Fig. 6 represents a section of the apparatus when thus in use (also slightly reduced in size). The numbers in the two figures correspond. In addition, on Fig. 6, A is the objective, B the slide carrying the object, and C the immersion fluid.

I have found this apparatus exceedingly convenient for the purposes of photo-micrography and sunlight work generally; for when I have once obtained any particular result by means of a certain obliquity, I am able to reproduce the effect at pleasure without any loss of time. It has also proved useful, for the same reason, by ordinary lamplight. When, however, the object of the microscopist is merely to resolve Amphipleura pellucida or similar tests mounted in balsam, by lamplight, with suitable objectives, I still give preference to the simple substage prism I described last year, through which I can throw the light at once at an angle of 45° by means of the concave

*

* See this Journal (1878), p. 246.

mirror or a small bull's-eye, and thus obtain for this particular purpose equally good effects, with less expenditure of time in making the adjustments."

Improvements in Microphotography."-Dr. E. Cutler describes the apparatus he adopted for photographing with Tolles' objective, the special features in which the apparatus differs from Colonel Woodward's plan (besides portability) being (1) in the size of the

* Am. Journ. Sci. and Arts,' xviii. (1879) p. 93.

condenser, and (2) the absence of the ammonio-sulphate of copper or alum cells, which are troublesome.

The condenser consists of an 18-inch Voigtlander photographic objective, about 3 inches in diameter, and is probably the largest ever employed in microphotography. The reason of its selection was simply to avoid heat. It is easy to see that if a 2-inch condenser is regarded as sufficient, the same amount of light could be obtained with a 3-inch, away from the heat focus and thus avoid the effect of focussing the sun's rays on the object and the objective. This practical point has been of great value, and explains the absence of contrivances to prevent the passage of destructive heat.

Modern Applications of the Microscope to Geology.*—An interesting article on this subject is contributed by M. L. Fouqué to the 'Revue des Deux Mondes.' The progress of human knowledge, he says, is not accomplished in a regular and continuous manner, but by starts. Sometimes a man of genius gives a new impulse to science by the power of the divine reflex which animates him, but more often, particularly in experimental researches, each clearly marked impulse of the scientific movement is signalized by the employment of a new method of investigation. Thus the invention of the Microscope was the point of departure of brilliant discoveries in natural history, and each of its improvements corresponded to a period of progress in the development of the science to which it was applied. To-day the manufacture of the instrument has arrived at a remarkable degree of perfection, its magnifying power is enormous, its images are of an extreme clearness, and ingenious arrangements have rendered the instrument more manageable without having lessened precision, and its constructors have known how to adapt it to the special requirements of each class of research.

The consequences of these innovations were soon manifest. The study of organized beings took an unexpected turn, anatomy and vegetable physiology were entirely transformed, the domain of the zoological sciences was enlarged beyond conception, and the secrets of life have been explored in their most mysterious functions.

The application of the Microscope to the examination of the inorganic world took place more tardily in consequence of special obstacles. These difficulties are now happily surmounted. A harvest of new results is being reaped, so rich that it dazzles the imagination of those who gather it.

In the first part of the article is traced the historical development of modern "Microscopical Petrology," commencing with 1858, when Dr. Sorby's memorable researches first appeared, and on whom is passed a warm eulogium as "the real initiator and propagator of the new method," and after dealing with the labours of Zirkel, Vogelsang, and Rosenbusch the author regrets as a "curious matter and one difficult to explain that though in Germany microscopical petrography is now studied with unequalled ardour, in England, the country of its origin, it seems to make but slow progress."

6

* Revue des Deux Mondes,' xxxiv. (1879) pp. 406–31.

The second part explains the results obtained by the application of the Microscope to the study of minerals and rocks, and particularly the light thereby thrown on the actual constitution of the latter, and on the complex structure of a great number of crystals supposed to be simple, their mode of formation and the changes in the temperature, chemical composition, and stability of the media during the process. The Microscope is thus able to give an account of the conditions which prevailed when the minerals were being formed, as well as doubling the field to which geology can extend its conquests.

The third part gives a summary account of the methods of examination by polarized light, and the modern improvements which have been made in the examination of minerals by its means.

Adams' Measuring Polariscope.-This consists of three principal parts. The lower section consists of a mirror, a lens, a Nicol's prism, and two other lenses. The upper section consists of lenses and Nicol's prism arranged in the reverse order. Each lens and Nicol's prism is supported separately by screws, and its position can be altered independently of the others. These two parts form a complete polariscope.

Besides these there is a middle piece, consisting of two lenses (nearly hemispheres) forming a box to enclose the crystal immersed in oil, their curved surfaces being concentric. The whole middle piece is supported on the tubes of the upper and lower portions, and may be turned about the optical axis of the instrument. The vertical graduated circle carrying the central lens and crystal may be turned through an angle about its horizontal axis. By means of an arc fastened perpendicularly on the graduated circle, with its centre at the centre of curvature of the central lenses, the crystal may be turned about another horizontal axis at right angles to the former, so that the crystals and the central lenses can be turned about each by three axes which are mutually at right angles. By means of a system of toothed wheels in gear with the rims of the central lenses, the central and crystal lenses may be turned separately about the optical axis of the instrument, so as to bring the planes of the optic axes of a biaxial crystal parallel to the plane of the vertical graduated circle.

Homogeneous Immersion.-From conversations which we had with microscopists at the time of Professor Abbe's recent visit, it appears that the difference between the modern "Homogeneous

Immersion" and the "Oil-Immersion" of Amici and Hartnack has not been appreciated.

One of the leading points of Professor Abbe's theory of 1874 was his explanation of the important bearing which the diffraction pencils have on the formation of the microscopic image so that the resolving power of an object-glass is dependent upon the diffraction pencils that are taken up by it.

This fact was not previously known, and in the absence of that knowledge it is not surprising that those who suggested the use of oil instead of water abandoned it in practice, not thinking it worth while