D
FIG. 47.

being placed due south, the end of the board upon which the heliostat stands is cut off at the angle corresponding to the glass of the

The window at which I work faces about S.E., and has the sun from early morning until about two P.M., and, to ensure the apparatus

window, so that the table can be easily placed exactly in the required position.

The table itself (fig. 48) is heavy and solid, and stands upon three legs, so as to secure an equal bearing. It is at such a height that the horizontal Microscope-tube is at a convenient level for eye-observation, when the observer is seated, so that all the preliminary adjustments, as regards cover-correction, &c., can be comfortably made, and the illumination regulated, before the camera is attached. The base-board of the camera pivots on a steady tripod, and can, during this process of adjustment, be swung aside out of the way, but be brought round when required, and the anterior end of the base-board then fits to the edge of the Microscope table. The attachment of the camera to the Microscope is effected in the usual manner. For my own work, I find it most convenient to use a camera of fixed length, viz. one metre from eye-piece to sensitive plate; but a bellows body, capable of variable extension, can, of course, be substituted if desired. The focusing rod disconnects at the anterior end of the camera, sliding back off a square pin from the portion attached to the Microscope table. It works by means of a string, that passes round the milled head of the fine-adjustment (Fig. 49). The bar which carries the socket of the substage condenser has attached to it a small platform, upon which can be placed a screen of dark-blue glass, to subdue the glare for eye-observation, or a small cell containing ammonio-sulphate of copper or other solution, for producing monochromatic light.

So far, however, I cannot say that I have experienced any practical advantage from monochromatic light. It appears to me that when ordinary sunlight is used, the blue-violet rays are so prepotent in their actinic power that they do all, or nearly all, the work, and the other rays have not time to produce any material effect. The supposed advantages of monochromatic light are then practically attained without any special means, unless, indeed, some special method can be devised for working with rays of shorter wave-length than the blueviolet; and any suggestion for accomplishing this I shall be glad to receive, and to give it a trial.

The resolving power of our objectives depends not only upon their numerical aperture, but also upon the wave-length of the light used; and the high ultra-violet rays should therefore give a higher resolving power than the blue-violet; but I have not yet succeeded in making them operative in practice.

As regards general manipulation, the only special recommendations that I have to make are :-(1) That the cone of illumination should always be strictly axial. (2) That the image of the sun should be focused exactly in the plane of the object, so that it shows sharp and clear on the ground glass when the object is in focus. Clouds close to, or passing across the face of the sun, should be seen almost as if a landscape lens was being used. (3) That no unachromatized lens

FIG. 48.

should be introduced in any part of the system. I cannot, therefore, advise the use of a bull's-eye between the source of light and the sub

stage condenser. The angle of the cone of illumination which gives the best results, varies, I believe, not only with the object, but also with

the individual objective used. Too narrow a cone is apt to canse diffraction fringes, too wide a cone produces haze. I have not had much experience in photographing test diatoms, but so far as it goes, I find that my own 2 mm. Zeiss Apochromatic, 1.4 N.A., gives its best definition of such objects when about two-thirds of its back lens is filled by the dioptric beam.

I trust this description of my apparatus will enable others who may be desirous of using sunlight illumination to adopt it, and, I hope, improve upon it. I shall be pleased to answer any inquiries as to any point that may not have been made sufficiently clear.

SUMMARY

OF CURRENT RESEARCHES RELATING TO

ZOOLOGY AND BOTANY

(principally Invertebrata and Cryptogamia),
MICROSCOPY, &c.,

INCLUDING ORIGINAL COMMUNICATIONS FROM FELLOWS AND OTHERS.*

ZOOLOGY.

A. VERTEBRATA:-Embryology, Histology, and General.

a. Embryology.t

Inheritance of Acquired Characters.t-Dr. J. F. van Bemmelen has written a detailed history of opinions and theories in regard to heredity, with special reference to the problem of the transmissibility of individually acquired characters. After a brief sketch of Weismann's position, he reviews with great completeness the relevant literature. Beginning with Hippocrates and Aristotle, he passes to Buffon and de Maillet, Robinet, and Bonnet, and thence to Lamarck and the "Transformists." The opinions of modern naturalists are classified according to the predominance of anthropological, physiological, and pathological considerations. Scholarly as the record is, we find some serious omissions, as, for instance, of Brooks and (ialton.

Studies in Mammalian Embryology-The Placenta.§-Prof. A. A. W. Hubrecht describes the placenta of Erinaceus europæus, and discusses the general history of placentation.

Its

I. Development of Yolk-sac and Allantois.-The youngest blastocyst observed had the form of an oblong sac, and measured 1/10 mm. outer wall inclosed a few aggregated cells the future hypoblast. The wall soon becomes more than single-layered, and exhibits an internal projection at the "anti-mesometrical" pole. Rapid growth thins out the wall of the blastocyst into a unicellular layer, with lacunar spaces containing maternal blood, and with numerous villiform processes from the columns intervening between the lacunæ. From the thickened polar epiblastic knob, the germinal area is formed by the separation of an

The Society are not intended to be denoted by the editorial "we," and they do not hold themselves responsible for the views of the authors of the papers noted, por for any claim to novelty or otherwise made by them. The object of this part of the Journal is to present a summary of the papers as actually published, and to describe and illustrate Instruments, Apparatus, &c., which are either new or have not been previously described in this country.

This section includes not only papers relating to Embryology properly so called, but also those dealing with Evolution, Development, and Reproduction, and allied subjects.

De Erfelijkheid van Verworven Eigenschappen,' 8vo, 'SGravenhage, 1890, pp. xiii. and 279. § Quart. Journ. Mier. Sci., xxx. (1889) pp. 283-404 (13 pls.).