TIIE ILLUMINANT-DAYLIGHT.
Ordinary daylight, especially that re flected from a white cloud, is to be preferred for general microscopic work, but is of little or no use for photo micrography, because it has not suf ficient intensity. Direct sunlight, how ever, is the best illuminant if it can be depended upon, which unfor tunately it cannot be in this country, and therefore other methods of illumination are more often employed. Dr. Wood ward, of America, to whom we are in debted for much valuable information on this subject., has given a description of the method which he employed of illu mination with sunlight reflected from a plane mirror fixed to a heliostat, and from this we take the following particulars : The photomicrographic apparatus is fixed on a shelf near to the window of the dark room ; the heliostat (Fig. 675) is placed out side the window, which is darkened with the exception of a small aperture, in which is fitted a lens mounted in a suit able tube. The lens employed is an
achromatic combination about 2 in. dia meter, and of about 10 in. focal length. By suitable arrangement of this lens the solar rays are brought to a focus, and be gin to diverge again before they reach the lowest glass of the achromatic con denser. In this way the solar heat is almost entirely removed, and it has been found that light could be obtained suf ficient to give distinct vision and faultless definition upon a cardboard screen with a power magnifying 5,000 diameters, while the heat was so slight that the drop of water placed between the immersion lens and the cover glass did not require re plenishing oftener than once in two hours.