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Mr. F. L. 0. Wadsworth on 



have the original half-prism form of Littrow, fig. 5, which 

 is now seen to be only one special form of a general class. 

 It is evident that by varying the angle a we may make the 

 angle (S anything we please from to 360°. Four special 

 cases only are considered : — 



(1) When /3 = 90°, then for 6=4,5° we have a=22i°. The 

 arrangement of the parts is shown in fig. 4. 



(2) When /3 = 180°, « = i<9, and we have the Littrow half- 

 prism arrangement already referred to (fig. 5). 



(3) When /3 = 270°, for = 45°, « = 112i°, and we have 

 the arrangement shown in fig. 6. 



(4) Finally, when /3=0 the reflected ray is parallel to the 

 coincident, and now «=— 0/2 (fig. 7). 



In all these last forms the prism-mirror system is mounted 

 directly on the graduated circle of the spectrometer and 

 revolves with it. Since the angular rotation of the prism 

 is exactly one half that of the refracted ray which passes 

 through it at minimum deviation, it follows that we may 

 determine angular deviations from the readings of the spectro- 

 meter circle, just as in the ordinary forms of the instrument, 

 except that the angular readings must be multiplied by 2 to 

 obtain the angular deviations. The movable parts of the 

 instrument (the mirror and prism) are therefore rendered 

 extremely light and mobile, without any sacrifice in accuracy, 



