A treatise on the kaleidoscope / By David Brewster.

  • Brewster, David, 1781-1868.
Date:
1819
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on the kaleidoscope / By David Brewster. Source: Wellcome Collection.

    125/208 (page 107)
    their breadth in the same proportion. Let us sup- pose, for example, that wlien the reflectors arc five inches long, we can safely employ an aperture one- fourlh of an inch in diameter ; then if the plates are made ten inches long, we may use an aperture two-fourlhs in diameter, the symmetry continuing as complete, and the light of the reflected images as intense, as when their length was only five inches. By increasing the length of the reflectors, therefore, we increase also the quantity of light; but, unfortunately, this increase of length is unfa- vourable to the other properties of the instrument; for we must now use a lens L L of a great focal length, which will render it necessary to receive the imao’e at a g-reat distance from the instrument. In order to render the effect as brilliant as pos- sible, the inclination of the reflectors should, in the present case, never exceed 30°, and might be even 36° or 45°. The objects should be selected as thin as possible, and none with dark tints ought to be admitted into the object plates.