James J. Furniss

“ When the bounding edges of the ribbon of variegated light which forms the spectrum are seen sharply defined at the same time that the lines across the spectrum are distinct, the adjustment is pretty correct; when such is not the case, the slit and lens are not in their proper relative positions. The narrower the slit, consistent with the necessary supply of light, the finer and more distinct are the lines. A black cloth, or a pasteboard box, with suitable apertures for the collimator and telescope tubes, should be placed over the prism to shut out all extraneous light. ”

“ Concerning the observing telescope, perhaps Proctor's remarks about a finder for an astronomical telescope may be repeated: "It will be easy for the student to construct one for himself, and will be a useful Fig. 2. exercise in optics." But in case the student may not want to take the trouble, he will be glad to remember that an ordinary pocket telescope or spy-glass may be purchased ready made for a dollar or two, and by removing the erecting lenses a small astronomical telescope may be produced which, as the magnifying power required is small, will answer every purpose of the beginner. ”

“ Focus the telescope for distant vision, align the telescope with the collimator, and move the slit till it (the slit) is seen distinctly in the telescope; then put the prism in place approximately, and move it round its axis until a position is found where (supposing the light of the sun or diffused daylight to be under examination) the lines in the green part of the spectrum are seen at their best advantage. It must be remembered that a slight change of focus is necessary for every color. ”

“ He who possesses and can spare the requisite means, will naturally provide himself with an instrument from the optician; but there are no doubt many who, while taking a great interest in this and kindred subjects, are so circumstanced that their outlay for scientific purposes must be limited to a very small sum. ”

“ 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 ”