500 CONSTRUCTION OF A SPECTROSCOPE. 



drawing a knife along a small ruler ; the cut must be parallel to 

 the longer sides of the glass, and equally distant from either. When 

 the glass plate is held against the light, the cut should appear as a 

 fine line, everywhere equally bright. The glass is now attached 

 with glue over the rectangular opening in the side of the box, by 

 means of four small strips of paper ; the side of the plate with the 

 tinfoil being turned towards the interior of the box. 



The box must evidently be of the proper size for allowing the 

 prism P to be in the position shown at the figure B. If the prism 

 has the size assumed in fig. 277, the box must be 6 cm high and 8 

 wide. The angles of inclination of the slanting side may be taken 

 from fig. J5 with sufficient accuracy. The length of the box should, 

 if possible, be 40 cm ; if it is shorter, the spectrum will not be so 

 long. 



The aperture for the eye is not in the middle of the slanting side, 

 but somewhat to the right of the centre. The proper position for 

 it is found in the following manner. The box is closed, with the 

 lid and the prism placed upon it about in the same position in which 

 it will be afterwards within the box, and close to the slit, at the 

 same height as the prism, a small burning candle is placed. When the 

 eye is brought near to the prism the flame is seen to be drawn out 

 into a spectrum with indistinct outlines. By slightly turning the 

 prism to the right or to the left the position of minimum deviation 

 is found that is, the position in which the spectrum is least dis- 

 placed to the left. The eye is then removed a few decimetres from 

 the prism, taking care to keep the spectrum in view. If the prism 

 is small, or the eye rather distant from it, only a portion of the 

 spectrum can now be seen. Now move the head sideways so as to 

 find by trial where the eye must be placed in order to see the yellow 

 portion of the spectrum exactly in the middle of the refracting sur- 

 face of the prism : the aperture must be made in that part of the 

 slanting side of the box which in this position of the eye is just below 

 the middle of the refracting surface. It should be cut with a sharp 

 knife, 15 mm square, as in the figure, or circular with a cork-borer, 

 15 mm in diameter; it should be 3 cm from the bottom of the box, 

 when the latter has a height of 6 cm , or if the box has a different 

 height from that assumed midway between top and bottom. 



The prism is now placed in the box as in JB, and a burning candle 

 or lamp is placed 4 or 6 cm just behind the slit ; the slit is next viewed 

 through the aperture, and the prism turned to the right or left until 

 the less bright, but better defined, spectrum of the light which 

 passes through the slit appears least deflected to the left. The 

 position thus found by trial is marked by three strips of pasteboard, 



