156 LIGHT AND PROTOPLASM [Cn. VII 



Extreme ultra-violet \ = 0.295/*; 1010 x 10 12 vibrations per 

 second. 



To obtain monochromatic light from the spectrum, REINKE'S ('84) spec- 

 trophor will be found useful (Fig. 41). In this instrument a beam of sun- 

 light cast by a heliostat through a slit at H is converged by means of an 

 interpolated lens, 0, upon a prism, P, set at the angle of minimum devia- 

 tion. Passing through this prism the rays are dispersed and a spectrum is 

 formed upon a diaphragm D. D l composed of halves bounding a second slit 

 whose position and width may be varied at will so as to include any desired 

 part of the spectrum. A large lens C known as the collector brings the 

 rays which have passed through the second slit to a focus at E. Just in 



FIG. 41. Diagram showing the 

 construction of REINKE'S 

 spectropbor and the path 

 of the rays in it. H, slit 

 next to heliostat ; O, pro- 

 jecting lens ; P, prism ; S, 

 Si, scale marked with wave 

 lengths ; D, D, diaphragm, 

 including a variable slit ; c, 

 Cj, collecting lens ; E, posi- 

 tion of object subjected to 

 the rays. The spectrum 

 ranges from A = 0.75 ^ to A = 

 40 F. (From REINKE, '84.) 



front of the diaphragm is placed a scale S, 5^ of wave lengths fitted to the 

 spectrum obtained. Such a scale may be constructed with reference to the 

 position of FRAUENHOFER'S lines by interpolation from Fig. 40.* To in- 

 clude rays whose wave lengths differ by exactly 0.05 /j. the slit must be wider 

 when at the blue end than when at the red end of the spectrum (Fig. 40). 



to form a hollow, triangular (60) prism. The plates are fixed to each other 

 and to the glass base by a pasty cement made by mixing plaster of paris and 

 liquid glue. This cement soon hardens, and is not attacked by the carbon 

 disulphide. The hollow prism is now filled with fluid, and a triangular glass 

 plate is cemented on as a cover. 



* If artificial light is used, two points on the scale can be obtained as follows : 

 Volatilize in a Bunsen flame, temporarily replacing the lamp, a salt of barium 

 and one of calcium, and note the position of the extreme blue band on the former 

 (line F) and the yellow band of the latter (line D). After determining these 

 two points the remaining lines can be plotted upon the scale. 



