the prism, (2) the collimator, (3) the telescope, and (4) the stand. The prism is the most important part of the instrument, and also the most expensive; but, as so much depends upon the performance of the prism, a good one obtained at the first will prevent the disappointment which inevitably follows the attempt to use a cheaper and less perfect article.
A hollow prism filled with a liquid such as oil of cassia, or bisulphide of carbon, may be used, or a flint-glass prism, or one of crown glass. A crown-glass prism may be procured for a comparatively small sum, but its dispersion is small, and to obtain really satisfactory results a train of three or more prisms is necessary, and such a number would be difficult for the beginner to handle without the automatic arrangement to be found in the regular instruments. The price of the hollow prism is a little higher than that of the crown-glass prism, but not quite so high as the price of one of flint glass; of its performance I know nothing by experience, but Lockyer does not speak very favorably of it. Perhaps on the whole the most satisfactory, and consequently in the end the cheapest article, is the flint-glass prism.
The collimator is a tube carrying at its outer end the slit, and at the end next the prism the collimating lens. The tube should consist of two pieces, one sliding easily within the other; so that the distance of the slit from the lens may be regulated. That distance should be equal to the focal length of the lens, in order that the rays of light passing through the slit in diverging pencils may be rendered parallel and sent through the prism as a cylindrical beam. Fig. 1 shows the collimator—A being the position of the slit, and B the position of the lens.
Fig. 1.
The focal length of the lens may be obtained near enough to give the approximate length of the collimator tube by projecting the image of some distant object sharply and distinctly on a screen, and then measuring the distance between the screen and lens. After the instrument is completed the adjustment of the lens and slit with regard to each other may be perfected by the following operation: Remove the prism, and bring the telescope, which must previously have been focused for distant objects, into line with the collimator; then move the sliding tube carrying the slit in or out until the image of the slit is seen sharply defined in the field of view of the telescope. The distance then between the slit and lens is equal to the focal length of the lens. The diameter of the lens need not be quite equal to the width of the refracting face of the prism. An ordinary convex lens of eight or ten inches focus, which may be purchased for a small sum, and which may easily be set in the tube by the student himself, will answer his purpose as well as a much more expensive article.
The figure of the slit is of great importance, and for fine work a
