A HOME-MADE SPECTROSCOPE. 809 



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 disappoint- 

 ment 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 bisul- 

 phide 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 favor- 

 ably of it. Perhaps on the whole the most satisfactory, and conse- 

 quently 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. A B 



The focal length of the lens may be |] \_I,,. .. .,-, -.___ J 



obtained near enough to give the approxi- 

 mate length of the collimator tube by -T ICJ- 1 



projecting the image of some distant ob- 

 ject sharply and distinctly on a screen, and then measuring the dis- 

 tance 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 carry- 

 ing 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 



